The Soviet PSS semiautomatic pistol is a six-shot semiautomatic silent weapon designed to utilize only the SP4 captive piston silent ammunition.  The PSS firing mechanism is single/double action with open hammer and slide-mounted safety/decocker of conventional Makarov design.  The sights are fixed, and the intended operational range of the weapon system is short.  Although approximately 25 years old, it remains on the forefront of captive piston technology and is still an operational weapon.

The SP4 (also referred to as 7.62x41mm) cartridge is unique.  The cartridge case is made from steel (alloy presently unknown) with a heavy copper-zinc plating meeting the specifications of C220 (ComBz).  The projectile is a 155-grain mild steel cylinder that is launched at approximately 650 ft/sec.  The projectile features a brass driving band on the forward diameter that engages the rifling in the short barrel.  There is a recess in the center of the rear of the projectile that both centers the rear of the projectile on a protrusion on the short piston as well as stabilizes the piston during its travel forward.

SP4 (7.62x41) sectioned cartridge showing relation of the projectile, driving piston, and cartridge case.

On firing, the primer ignites a small powder charge that then accelerates the piston forward (pushing the projectile).  At the end of the piston’s travel, it is suddenly arrested by a retaining crimp or shoulder on the front of the cartridge case.  The projectile then continues in a forward direction, and the case ejects from the weapon.  The driving piston, which is captured in the case, obturates the neck of the case and contains the pressure within the spent cartridge case.  It is known that these expended cartridges may contain pressure for several weeks, and attempts to disassemble them within a month of firing can be hazardous.  Because the driving gases are contained, the weapon is essentially silent.  Sound measurements utilizing the International Industry Reference Protocol have shown the absolute sound pressure level to be in the vicinity of 122 dB, which is on a par with simple airguns and suppressed .22 LR rifles.

We have had the opportunity to do fairly extensive testing and study of this unique weapon system.  This includes complete disassembly of the PSS pistol, a project not to be undertaken lightly.  The pistol includes a floating chamber, and the method of operation and reason for the floating chamber have been a matter of speculation until recently.

The floating chamber is not a new concept and dates back to the days of its inventor, David (“Carbine”) Williams around 1931.  The floating chamber is a steel cylinder that surrounds the cartridge case and that can move rearward with the case in the early part of the weapon’s unlocking cycle.  It has most commonly been used in .22 rimfire weapons.  In this application, it permits a greater rearward force against the face of the bolt to assist in cycling the weapon by presenting a larger area with greater mass to drive rearward against the bolt face.  Proper operation of the floating chamber for recoil enhancement requires that high pressure propelling gases be present in the bore of the weapon between the forward end of the cartridge case (and floating chamber) and the bullet in the bore.  This permits propelling gases to act against both the cartridge case and floating chamber.  The purpose of the floating chamber in the PSS has been a mystery.  The author believes that the sequencing shown by the high speed video offers clues as to its function.

In smaller caliber handguns and submachine guns, the most common action is the simple, unlocked blowback.  In these weapons, ignition of the powder charge in the cartridge generates significant pressure that exerts force in all directions.  The forces directed forward press against the base of the relatively lightweight bullet, accelerating the bullet forward through the barrel.  Being lighter than the weapon, the bullet accelerates to a high velocity.  The acceleration continues as long as pressure remains in the bore of the barrel until the bullet exits and pressure suddenly drops.  This is simple Newtonian physics of equal and opposite reaction: the energy to drive the lightweight projectile forward at high velocity is equal to the energy to drive the heavier slide or bolt rearward a shorter distance and at a lower velocity.

In discussing weapon cycling, time is often difficult to conceptualize.  In conventional weapons, there is a finite (although short) time under pressure referred to as “dwell time.”  This is the time during which the bullet is still being propelled by the gases in the bore and there is pressure in the bore to act on the slide or bolt.  If a floating chamber is present, the pressure will initially move the cartridge case (and floating chamber) rearward almost microscopically and allow pressure to push on the larger cross section area of the floating chamber.  In a locked breech handgun, the barrel (locked to the slide) starts rearward acceleration and the action starts to open once the barrel unlocks from the slide and pressure drops in the bore and cartridge case.

In conventional weapons, pressure in the cartridge case causes the case to expand slightly and form a gas seal against the chamber itself or the neck of the chamber, maintaining pressure in the bore.  The same forces driving the bullet act over the same finite time interval against the base of the cartridge case, which starts acceleration of the case and slide (or bolt) rearward against the recoil spring with the same energy.  Because of the greater mass of the slide or bolt, velocity is significantly lower.  However, friction of the expanded cartridge case against the chamber delays actual movement for a few microseconds until the pressure drops slightly on bullet exit.  At that point in time, the case can start its extraction process.  In high speed video analysis, there has been shown to be virtually no extraction until the projectile has exited the bore.

FRAME 683: The hammer has completed its fall, but there is no observable movement of the slide or exit of the projectile, which is still within the barrel. It is assumed that ignition has taken place and the piston is starting its travel forward. FRAME 684: The projectile is approximately 70% out of the end of the barrel with no sign of instability. The copper driving band is no longer engaged in the rifling. The slide has also started its rearward travel, and the rear of the floating chamber is in contact with the face of the slide. FRAME 685: The projectile has cleared the muzzle of the piston and the rear of the projectile is approximately three projectile lengths forward of the end of the barrel. There is a small area of turbulence from escaping gas between the rear of the projectile and the front of the barrel. It is thought that this is a small amount of propelling gas leakage before the piston completely obturates the cartridge case and that this is what is responsible for the sound level to be as high as 122 dB. The slide has continued its rearward travel with the floating chamber moving rearward with the slide. The slide and cartridge base are just beginning to move rearward slightly faster than the floating chamber. FRAME 686: The projectile has continued to move forward another two to three projectile lengths and remains stable. The small puff of escaping gas is dissipating. At this point, the slide continues its rearward travel. The rearward motion of the floating chamber continues, but not as fast as the slide and the base of the cartridge case starts to be visible. FRAME 687: At this point, the slide continues to open with the base of the cartridge case in contact with the bolt face. Although the floating chamber continues its rearward motion, the gap between it and the slide face widens. The projectile remains stable. FRAME 689: The floating chamber now remains stationary with respect to the pistol’s frame and the slide continues rearward extracting the cartridge case. At its farthest, the floating chamber appears to have moved approximately four millimeters. FRAME 691: As the slide and cartridge case continue rearward, the floating chamber starts to retract forward into the barrel. FRAME 697: Continued rearward slide movement. The floating chamber is now completely retracted into the rear of the barrel into its original location. FRAME 701: The front of the cartridge case has been completely extracted from the barrel and is ready for ejection.

In late March 2012, we utilized an Olympus I-Speed 3 high speed video camera to observe the operation of the pistol in operation.  Camera settings were a frame rate of 3,000 frames/second and a shutter speed of four microseconds (1/250,000 second).  This corresponds to a frame every 333 microseconds.  The short shutter speed allowed a series of frames capturing the projectile in early flight to check for yaw and other signs of instability.  In operation, the camera records continuously, looping through memory.  To capture the action (in this case weapon cycle), the camera is stopped manually immediately on completion of the event, and the desired segment of the video showing the action is saved as a video file.  One of the options in the video software is to print individual frames as a series of JPEG still images.  Nine of these images are presented with the first five being consecutive.

In the case of the SP4 captive piston ammunition, all of the projectile propelling gas is contained within the cartridge case and there is no propelling gas to create pressure in the bore of the pistol prior to projectile exit from the barrel of the PSS pistol.  There is also no gas to drive the floating chamber rearward, initially raising the question as to why it was included in the PSS pistol.

The series of frames from the high speed video starts (F-683) with the hammer having fallen and ignition.  The next consecutive frame (F684) shows the immediate start of rearward movement of the floating chamber pressing against the slide.  It isn’t until the fourth frame in this series (F-686) that the cartridge case starts to extract from the floating chamber.  It is interesting to note that in spite of minimal rifling length, a cylindrical projectile, and the awkward propulsion method, there is no apparent instability or yawing of the projectile.

The floating chamber in the PSS has several functions.  For one, it increases the mass of the moving parts in the initial stages of recoil.  There has been speculation that it also is used to slow down the slide on its final stages of movement to dampen the sound of the slide hitting the stop on the end of the recoil cycle.  However, the high speed video does not bear out this explanation.  What the floating chamber does do is to eliminate the lag time waiting for the cartridge case to shrink slightly away from the chamber wall.

PSS pistol field stripped. The floating chamber in the rear of the fixed barrel is labeled.

At the instant of ignition, there is going to be a minimal amount of expansion of the steel cartridge case.  Initially, the pressure in the rear of the cartridge case is going to be higher than the pressure elsewhere in the case, and this is going to cause increased expansion of the case in the rear.  Once the piston has reached the extent of its travel, pressure will equilibrate and there will be some relaxing of the expansion in the rear of the case.  The expansion of the rear, while only transient, will increase wall friction significantly until pressure equilibrates.  The dwell time in this weapon is significantly shorter than in a more conventional blowback, because the dwell endpoint is when the piston stops as it is captured by the end of the cartridge case.  Piston travel is approximately one inch.

As the piston moves forward driving the projectile, there is high acceleration until the copper driving band engages the coarse rifling in the barrel within the first 2-3 millimeters of piston travel.  The expanding gas inside the cartridge case causes force in opposite directions: first to propel the projectile toward (and out) the end of the barrel, and second to start accelerating the base of the cartridge case rearward against the slide.  Since pressure remains high in the rear portion of the cartridge case, the case remains stuck in the floating chamber until the piston is at the forward limit of its travel and pressure equilibrates.  Because the floating chamber shows little or no friction with respect to the barrel or frame, the slide can start immediate rearward acceleration.  Once the pressure in the case has equilibrated, the case can then start its extraction from the floating chamber, which then returns to its original position under spring tension.  The time under pressure for extraction is significantly less than in conventional firearms because of the limited motion of the piston, and any delay in initiating rearward travel would prevent complete cycling of the weapon.

Were it not for the floating chamber, cycling of the weapon would not be possible as friction would hold the case in the chamber of the barrel too long.  Further, it is thought that the floating chamber provides additional reinforcing of the rear portion of the steel cartridge case until pressure has equilibrated.

There are several items of information that would be helpful in proving this theory. First, it would be of great interest to know the pressure curve in the cartridge itself.  Lacking ability to track the gradual change in pressure from ignition until the piston arrests, knowing the maximum pressure would be of value.

The forgoing is a postulation as to the mechanism of cycling and extraction of the SP4 cartridge in the PSS as well as the role played by the captive piston in this most unique weapon.

The design of a suppressor is not quite as simple as it may appear.  Not all suppressors are suitable for usage on all weapons.  While it should be intuitive that a suppressor designed specifically for .22 LR will not withstand the pressures of the 5.56mm cartridge and be destroyed, there is the temptation to try to make any 5.56mm suppressor work on any .22 centerfire weapon.  This may not be a wise choice.

In its simplest explanation, suppressors reduce sound by reducing the sudden release of pressure at the instant of bullet uncorking, usually by expanding the volume in the suppressor.  The pressure that the suppressor has to deal with is generated by the burning of the propelling gases, and the volume of gas generated is related to the amount of powder in the cartridge case.  Since the bullet is slightly larger than the bore, a lot of pressure is required to not only accelerate it through the barrel, but to sustain its velocity.

Depending on the burn rate of the powder, the pressure inside the bore propelling the bullet will either peak rapidly (as in rimfire and pistol cartridges) or more gradually in the case of centerfire rifle cartridges.  Here, the terms “rapidly” and “gradually” are relative – it is all very fast.  The author has performed actual pressure measurements in the bore of a 5.56mm rifle at various positions and times.  Some of that information will be the subject of another article.  What is germane to the current discussion is what the residual pressure is in the bore at the instant of bullet uncorking, because this is the volume and pressure of gas that the suppressor needs to address.

The first portion of the suppressor that has to contain the bore pressure is the entrance chamber, and this may well be the most critical chamber with regards to strength and safety.  Depending on the powder/gas load, barrel length, and entrance chamber volume, the pressure here can vary between several hundred and several thousand pounds per square inch.  If the entrance chamber pressure is too great for the structural design, the outer tube can weaken, bulge, or even fail.

For a given entrance chamber volume and pressure, the geometry can make a dramatic difference in integrity.  An important calculation is referred to as “hoop stress,” which is calculated by multiplying the pressure (in pounds per square inch) by the diameter of the chamber (to the middle of the chamber wall) and dividing by the wall thickness.  The units are pounds/square inch.

The safety factor is calculated by dividing the yield strength of the wall material by the hoop stress.  At a safety factor of one, 50% of the units will fail.  For suppressors, a safety factor of two is acceptable.  The aircraft industry requires a safety factor of 2.5 or better.  Most of the calculations are performed automatically when performing finite element analysis.  However, for FEA (finite element analysis) to work, one has to know the actual measured pressure within the vessel (suppressor entrance chamber or even in the barrel itself).  Unfortunately, attempting to simply calculate pressures in the beginning of a silencer based on bore volume, maximum SAAMI chamber pressure, and entrance chamber volume can easily lead to dangerously erroneous conclusions.  With one exception, the author knows of no suppressor company that actually measures direct entrance chamber pressures in the entrance chamber.

Pressure testing the entrance chamber of a 1.5 inch diameter quick detach 5.56mm suppressor on an M4 carbine with a 14.5 inch barrel. The pressure sensor is in the fixture surrounding the suppressor, and the charge meter shows an absolute pressure of 1,668 psi. Calculations of hoop stress show a safety factor of 3.1 at ambient air temperature and a safe 1.9 after 100 rounds fully automatic fire for this suppressor. The safety factors were also acceptable on the 10.4 inch barrel.

Rifle barrels are generally tapered from large over the chamber to thinner near the end. This is to have adequate wall thickness for safety over the higher pressure portions of the barrel and to reduce weight in the lower pressure far barrel.  Even at its furthest, the barrel wall thickness is significantly greater than the wall thickness in the entrance chamber of a suppressor.  Rifle barrel manufacturers have spent a lot of time making pressure measurements.

Marginal safety factors in a suppressor entrance chamber can often be easily corrected by several means.  Obviously, reducing the pressure by using underpowered ammunition is not viable if the weapon system is to be used for anything other than punching holes in paper.  One can also increase the barrel length, which may or may not be practical.

First is to increase the volume in the entrance chamber.  Increasing the diameter may not reduce the hoop stress simply because while the pressure is reduced, the radius is increased.  The answer here is to increase the chamber length (which has other positive effects on suppression).  Sometimes this can be accomplished by removing one baffle.

Second is to increase the wall thickness.  This has the definite disadvantage of increasing the weight, which is often a critical factor in the end user’s requirement.

The third option is simply changing to materials with higher yield strength, such as the change from 300 series stainless steels to 4130 chrome moly steel.  Series 300 stainless steels have yield strengths in the vicinity of 30-35 kPsi (thousand pounds per square inch).  This is the material that seems to be specified for most suppressors because its corrosion resistance is thought by many to be of importance.  It is certainly not the most structurally suitable material, and it is interesting to note that no firearms (especially barrels) are made from this alloy.  On the other hand, 4130 steel has a yield strength of 75-80 kPsi, better than twice that of 300 series stainless.  Simply switching to 4130 will better than double the safety factor without increasing weight or affecting heat dissipation.

Regardless of the material selected, suppressor heat buildup can adversely affect the safety factor.  Measurements of heat buildup in a suppressor have shown that in 5.56×45 NATO, a 100 round burst will raise the core temperature of a suppressor by approximately 750 degrees Fahrenheit above ambient air temperature.  On an average day, the 100 round burst will raise the temperature inside the suppressor to around 800 degrees.  At 800 degrees, most steels (including 300 series stainless) have 62% of the tensile and yield strength compared to room temperature.

The loss in yield strength after a 100 round burst can easily reduce what may have been a safety factor of 2 to a safety factor slightly over 1, at which point suppressor failure becomes more problematic.

The piezoelectric Kistler 6215 pressure sensor is screwed into either a fixture with a passage into the suppressor entrance chamber or into a sleeve welded on the vessel to be measured. With a maximum input of 100,000 psi, it is suitable for measuring bore pressures in a firearm, either in the chamber or in a barrel port.

Several years ago when this writer started doing pressure measurements, a question was raised about excessive instantaneous peak pressures in the entrance chamber of a relatively small (1-3/8 inch diameter, 6 inch long) 5.56mm suppressor built from 300 series stainless steel.  The suppressor was designed for, intended for, and rated by the manufacturer for the M4 carbine with a 14.5 inch barrel.  Users, however, were mounting the suppressor on 10.5 inch barrels, which raised some question as to suitability of this suppressor on this barrel length, especially since a number of users were training and employing significant fully automatic fire.

There are two recognized methods for measuring pressure in a vessel such as a silencer.  The least expensive method is the use of a strain gauge.  This is a sensor affixed to the exterior of the pressure vessel (usually with an adhesive) and connected to a recording instrument.  The gauge actually measures stretching of the vessel wall when pressure is applied, and it must be calibrated for each application.  Calibration is a relatively simple process of sealing the vessel and applying pressure to a fluid trapped in the chamber.  While this certainly can be done, it is a tedious and messy process and must be done for each sensor gauge and silencer.  Further, although the strain gauges are not expensive, they cannot be re-used and a new one is necessary for each silencer.  Few ammunition developers use this method, because it is not as accurate as direct chamber pressure measurements.

The other method is direct measurement of the pressure within the vessel.  This requires a high pressure sensor that has a direct connection to the volume within the vessel.  The sensor, a quartz piezoelectric transducer, is housed in a steel cylinder that threads either into the chamber directly or (far more commonly) into a housing surrounding the chamber.  The housing then has a port (hole) communicating directly into the chamber.  The signal pulse from the sensor is interpreted by a charge meter external to the system.  For chamber pressure measurements (ammunition development), the sensor is screwed into the barrel over the chamber and a hole is drilled into the cartridge case.  For a silencer, the sensor is screwed into a fixture attached to the silencer, and a hole is drilled through the wall of the silencer to provide the communication.  This is the most accurate method of pressure measurement.

A fixture to hold the piezoelectric pressure sensor was built and clamped around the suppressor over the entrance chamber, and a 2.5mm hole was drilled through the outer wall into the entrance chamber to permit actual pressure measurements.  The equipment used was a Kistler type 6215 high pressure sensor and a Kistler 5015 charge meter.  This was mounted first on a standard M4 carbine (14.5 inch barrel) and subsequently on an HK 416 (10.4 inch barrel).  In both cases, multiple rounds were fired and the pressures recorded.  The same ammunition (M855) was used for all tests.

The results were interesting.  When mounted on the 14.5 inch barreled M4 carbine, the pressure in the entrance chamber of this suppressor averaged 1,999 psi and the safety factor calculated to 2.8.  However, when mounted on the 10.4 inch barrel, the entrance chamber pressure averaged 2,998 psi (50% higher), and the safety factor calculated to 1.9.  While a safety factor of 1.9 is acceptable, users firing 100+ rounds of fully automatic fire in closely spaced bursts raised the temperature of the suppressor, resulting in de-rating the yield strength of the suppressor to the point where the safety factor was under 1.2, at which point the number of failures is going to increase.  While failures may not involve catastrophic rupture, there is the definite possibility of bulging (with further weakening) of the outer tube over the entrance chamber or parts going down range.

In this particular suppressor for use on a short barrel, the solution was simply to increase the length of the entrance chamber (reducing entrance chamber pressure) and change the structural material to something other than 300 series stainless steel.

Entrance chamber pressures in rimfire and in pistol caliber silencers are low and are rarely important in suppressor yield strength issues.  Even with inexpensive aluminum alloys like 6061, unless the wall thickness is ridiculously thin, the pressures are going to be low enough that safety factor concerns are non-existent.

This is not true for centerfire rifle calibers.  In these weapons, gas loads are high and pressures within the bore are significant, including at the instant of bullet uncorking.  The suppressor’s entrance chamber, while usually of greater volume than the weapon’s bore, must be able to contain these pressure peaks for an exceptionally brief (but finite) period of time.  With the goal of producing lighter and shorter suppressors, entrance chamber volumes are sacrificed resulting in higher pressures.  Coupling this with the propensity to use too-short barrels, there is less distance (volume) for pressures within the bore to abate, resulting in increased stress and pressure in the entrance chamber.  Both of these factors (along with the perceived necessity of using 300 series stainless steel and heating from fully automatic fire) increase the hoop stress resulting in decreasing safety factors to the point of possible suppressor structural failure.  It is this writer’s opinion that pressure measurements and safety factor calculations should be made in all rifle suppressors under the most adverse anticipated combinations of weapon configurations.

The handgun is an ideal candidate for suppression in instances where concealability of a quiet weapon with reasonable terminal ballistics is a goal.  With some exceptions, these suppressed weapon systems consist of a conventional handgun with an extended, threaded barrel to which a suppressor is attached.

Integrally suppressed handguns are fairly common in .22 rimfire, especially in the United States.  They are used for target practice, small game hunting, and occasionally for less honorable purposes.  As the OSS High Standard HD Military, they saw some action in the clandestine services.  They were also issued to pilots of the U2 spy plane used during the cold war.  Military Armament Corporation manufactured a number of integrally suppressed Ruger MK-1 pistols as well as the Ruger 10/22 rifle for use behind the lines in Vietnam.

For centerfire handguns, numerous muzzle attaching sound suppressors are available, both currently and historically.  While most attach by means of threads on an extended barrel, there have been a few effective quick-detach offerings.  All suffer the requirement of ammunition selection.  With the exception of the .45 ACP and the anemic .380 (9mm Kurtz), most ammunition is supersonic in a handgun to deliver maximum terminal effect.  Supersonic bullets generate a significant supersonic signature.  While this may not reveal the location of a shooter, it certainly alerts bystanders to the fact that a shot has been fired.  Velocity control, without special ammunition, usually requires that propelling gases be vented from the barrel before the projectile has had a chance to exceed the speed of sound.

The top weapon is the PB integrally suppressed pistol (slide open), and the lower weapon is the conventional issue model PM Makarov. The cosmetic and functional similarities are apparent when the two weapons are side-by-side. The button on the left side of the PB just behind the trigger guard, where western weapons would have the magazine release, is for disassembly of the grip. The magazine release is in the conventional Makarov location in the heel of the grip.

There have been a few (very few) handguns designed specifically to keep standard centerfire ammunition subsonic for suppressed use, and most were designed for covert, close range assassination weapons.  One of the older designs dating back to World War II was the British Welrod (a single shot, magazine fed, .32 ACP handgun) and its cousin, the .32 Sleeve Pistol.  The Soviets have designed several silenced handguns for similar purposes.  Other than the captive piston weapons (such as the PSS), one of the more unique is the model PB, often misidentified as “P6.”

It is believed that the PB was designed and initially built at Tula Arsenal in the USSR during the Cold War period in the mid 1960s and is credited to designer A. A. Deryagin.  Because of minor differences, it is thought that some were built by other factories, including in East Germany.  Some of the earliest surfacing of this weapon was in Afghanistan after the Soviet armies abandoned their failed Afghan experiment in the mid 1980s.  It appears that this weapon was issued to certain of the Spetnaz units.

The PB shares some cosmetics and functionality with the common issue Makarov PM pistol in caliber 9×18.  As with the standard Makarov, it is a double action semiautomatic blowback utilizing the standard 8-round Makarov box magazine.  The safety, like the PM safety, is located on the left side of the slide, and when engaged, drops the hammer on the locked firing pin.  The magazine catch is in the heel of the butt, and the trigger permits both single and double action firing.  This is where the similarities stop.  Unlike the PM, the PB’s ported barrel reduces the velocity of the 9x18mm round to approximately 290 m/s (951 fps) instead of the 340 m/s (1,115 fps) of the standard pistol.  While the overall appearance and operation is similar, it is a totally new pistol with the only interchangeable part being the magazine.  Few of the PB pistols have been seen in the Western world, and most that have surfaced are in government collections.  Little is known about this pistol and its exact origins.  Published data is scarce and fragmentary.

The field stripped Makarov PB shows a tang on the bottom rear of the secondary suppressor tube that engages the plunger in the front bottom of the primary suppressor tube to prevent unscrewing (1). The engagement that retains the primary suppressor tube to the frame is shown by the arrows (2). The small cutout in the primary suppressor tube that receives the stud on the trigger guard to prevent rotation of this tube is not seen on this photo.

The complete weapon has an overall length of approximately 310 mm (12.2 inches) and a height of 140 mm (5.51 inches).  The weight of the complete unit is reported to be 970 grams (2.14 pounds).  The suppressor consists of two parts: a primary suppression chamber built to surround the ported barrel and a separate, secondary muzzle suppressor.  The suppressor tubes are 32 mm (1.26 inches) in diameter.  While the suppressors themselves are not serialized, the host weapon frame carries the serial number in the normal Makarov location (left side, behind the grip).  The slide is also serialized, but may or may not match the frame.

The primary suppressor is built surrounding the barrel, and incorporates alternating rows of 3 and 2 ports in the specimen we examined.  There appeared to be no effort to align the ports with the rifling grooves, unlike many other Soviet suppressors using rifled barrels.  Reports of some variants of the PB include four rows of five barrel ports spiraling with the rifling.  The rearmost port is slightly over 40 mm (1.57 inches) from the bolt face and the last port is approximately 25 mm (0.98 inches) from the end of the barrel.  There is a collar that slips to the rear of the barrel, forming the rear mount for the primary suppressor tube.  The barrel does not protrude beyond the end of the front bushing, which contains interrupted female threads for attachment of the secondary removable suppressor.  The outer tube is retained by an interrupted groove engaging the front of the frame after a quarter turn.  The standard Makarov trigger guard disassembly post keeps the primary suppressor tube from rotating.  There is a spring-loaded latch on the bottom front of the tube to lock the secondary suppressor in place and prevent unscrewing.  The volume surrounding the barrel is filled with a roll of fine screen and a spring coil, the purpose of which is to absorb heat rapidly through its large surface area and to diffuse the gases escaping from the barrel ports.  The front sight is mounted on the front of the primary suppressor tube, and the rear sights are slightly higher than normal to accommodate the higher front sight.

Field stripped primary and secondary suppressors on the PB silenced Makarov. It is interesting to note that since the weapon system has two silencers, if it were to be registered in the United States, it would require two separate serial numbers and two separate registrations. On the original, the silencers themselves are not serialized. The host firearm is serialized in the conventional location (left side of the frame, behind the grip).

The slide on the PB has the appearance of being a standard PM Makarov slide that has had most parts forward of the bolt face and above the rails simply cut away.  Appearances are somewhat deceptive.  The weapon is a simple, unlocked breech blowback, and simply cutting away part of the slide would reduce slide mass and seriously increase rearward slide velocity.  Mass was added to the slide by increasing the length and thickness of the slide rails, and there was some lengthening of the frame.

In the PM, the recoil spring surrounds the barrel, as in a number of small pistols, such as the Walther PPK series.  This is obviously impossible with a suppressor completely surrounding the barrel.  Instead, the recoil spring was relocated to sit vertically in the grip behind the magazine well, and the grip was thickened to accommodate the spring.  The slide was coupled to the spring by a bell-crank on the right side of the magazine well.  One arm of this piece engages a cutout in the bottom of the right rear of the slide, and the other arm has a roller that compresses the spring.

The secondary suppressor is a more conventional Soviet muzzle design and is easily disassembled for maintenance.  The core of the suppressor consists of approximately five parts spot welded together: two sheet metal rails (top and bottom), the front cap, the rear mount, and the baffle stack.  The baffle stack is fabricated from a piece of sheet steel that looks like slanted washers with tabs to be spot welded to the top and bottom rails.  The flat baffles are set at approximately a 60 degree angle to the bore axis with the first and last baffles slanting the same direction and the middle one opposite.  The rails are welded to the front end cap and the rear mount.  Two small spring-loaded plungers in the front mount retain the suppressor tube on the baffle assembly.  The rear mount slips over the end of the barrel, and the interrupted threads permit tightening the suppressor onto the PB with 1/4 turn.  The plunger in the front mount of the primary suppressor keeps the secondary muzzle suppressor from unscrewing.

The PB with the primary suppressor outer removed showing the screen roll (surrounding the perforated barrel) and both the front and rear endcaps in place.

The PB was issued with a leather holster that stored the secondary suppressor separate from the weapon, which could be fired using only the primary suppressor.

Disassembly is fairly intuitive.  To remove the secondary suppressor, the locking catch on the bottom of the primary suppressor is withdrawn rearward and the secondary suppressor twisted 90 degrees counterclockwise to remove it from the interrupted thread.  As in field-stripping the standard PM Makarov, the trigger guard is depressed and the primary suppressor tube rotated 90 degrees counterclockwise.  At this point, the primary suppressor can be pulled off the barrel revealing the screen packing.  There is a button on the frame at the rear of the trigger guard (this is NOT a magazine release) that retains the one-piece grip.  Pressing this allows the grip to be removed rearward, revealing the bell-crank actuator and recoil spring.  Lifting out the actuator will allow the slide to be pulled to the rear and lifted off the frame.  The secondary suppressor can be disassembled by pressing the small spring-loaded plunger locking the outer tube to the front end cap.  The outer tube then removes to the rear.

Reassembly is pretty much the reverse of disassembly and is also fairly intuitive.  However, there are a few points to remember.  First, the trigger guard needs to be temporarily returned to lock the slide in position before replacing the bell-crank actuator.  Second, when re-installing the grip, it needs to be slid forward with considerable force.

On two separate occasions, we had the opportunity to perform standard sound measurements on the same Makarov PB integrally suppressed pistol.  Both were on the same weapon in the possession of a dealer in the United Kingdom.  The first set of measurements were performed in mid 2001 and the second three years later in the fall of 2004.  In both instances, measurements were performed at the reference location specified in Mil-Std-1474C (and D).  These consisted of placing the microphone 1 meter (3.28 feet) to the left of the muzzle, 90 degrees to the bore axis, and 1.6 meters (5.25 feet) above grass.  A-weighting was used.  A string of seven measurements were obtained, first with a standard non-suppressed PM Makarov, and then with the PB integrally suppressed weapon.  Ammunition was standard Soviet 9x18mm.  On both occasions, the environmental conditions were comparable, and we observed only minimal (and insignificant) variations between the two measuring sessions.

Located in the right side of the frame of the pistol under the grip, this bell-crank actuator couples the energy and force of the horizontal slide movement to the vertical recoil spring located in the back of the pistol grip. The tab labeled “3” rides in a cutout in the slide, the actuator rotates about a pivot pin (“2”), and the energy is coupled into the stiff recoil spring by a roller (“1”). The spring is retained when the 1-piece grip is installed.

One of the important calculations in evaluating a suppressed weapon is what is referred to as “first round pop.”  This is simply how much louder the first round is compared to the average of remaining rounds, and it is generated by a secondary burning of partially burned powder particles detonating within the suppressor filled with air (oxygen).  After the first round has been fired, there is no further oxygen left in the suppressor.  In most designs, this can be significant (well over 6 dB), and as little as 3 dB is obvious to the casual observer.  First round pop can be eliminated by a number of techniques, including purging the suppressor of oxygen prior to firing.  The majority of suppressors have a significant first round pop.  In covert use, the sound level of the first round is the only one that counts.

The non-suppressed average was 158.9 dB in 2001 and 158.2 dB in 2004.  This difference is considered to be meaningless.  The suppressed Makarov PB had an average sound pressure level of 127.5 dB for a net reduction of 31.4 dB.  Although the absolute sound level is comparable to a suppressed Beretta M9 (9x19mm) with a current design thread mounting muzzle suppressor, the lower velocity projectile from the PB generated a lower tone which was less noticeable.  We did not measure the sound level of the weapon with just the primary suppressor by itself.  Further, in both measuring sessions, the first round was only 1.5 dB louder than subsequent rounds – an important consideration in a weapon designed for covert usage.

In summary, the Makarov PB integrally suppressed pistol is a highly effective covert weapon.  It has a bulky appearance, but it is more compact than conventional pistols with a screw-on muzzle suppressor.  It consists of a special-design 9x18mm pistol and utilizes two suppressors on the same weapon.  The secondary muzzle suppressor is easily removed for concealability but can be re-attached quickly.  Although it can be used without the secondary muzzle suppressor, maximum effect is obtained when the weapon is used as designed.  It has the advantage of velocity control to keep standard issue ammunition subsonic.  While the lower velocity does slightly reduce the kinetic energy of the projectile, the soft sound signature is ideal for its intended purpose: close range assassination.

The author is indebted to the staff and personnel of the Firearms Technology Center of the Royal Armouries (UK) and its predecessor (MOD Pattern Room) for the opportunity to examine, disassemble, and photograph this weapon.

Many of us remember the finale in the 1965 movie Thunderball when James Bond and his allies battle Emilio Largo, the evil Number Two of SPECTRE (SPecial Executive for Counter-intelligence, Terrorism, Revenge and Extortion), and his cohorts, in the massive underwater battle scene where the main battle implements were knives and elastic powered or compressed gas spear guns.  Though that book, film and battle was fictitious, war beneath the waves isn’t.

The Russian company Tsniitochmash, the Central Scientific Research Institute of Precise Mechanical Engineering, located in the Moscow Region, has produced two firearms for the underwater warrior: the 4.5mm SPP-1M underwater pistol and the 5.66mm APS underwater assault rifle.

Surprisingly, these are not new weapons as they were developed and put in Russian service over thirty years ago.  They are excellent examples of the secrecy that surrounded the Soviet Union and were unknown, even at classified levels, in the West until Tsniitochmash began offering them publicly in 1993.

The results of firing in the air instead of underwater. The darts are destabilized rapidly and tumble, striking the target on the flat or folding into it. Very lethal, but not very accurate. (Dan Shea)

4.5mm SPP-1M Underwater Pistol
Developed in the late 1960s at the request of the Soviet Navy and accepted in 1971, the 4.5mm SPP-1 (Spetsialnyi Podvodnyi Pistolet – Special Underwater Pistol) was designed to arm combat divers (frogmen).  Later, the SPP-1 was upgraded to the SPP-1M that was basically the same pistol but had an extra spring above the sear to improve trigger pull and had a larger trigger guard to accommodate the use of diving gloves.  It is still in use by Russian Navy Special Forces.

The SPP-1M underwater pistol is a manually operated handgun that consists of four smoothbore barrels grouped in a square cluster.  The barrel cluster is hinged to the frame just in front of the trigger guard and breaks open in a similar fashion as a break-open shotgun.  There is a single striker (firing pin) and the double action firing mechanism fires one cartridge sequentially each time the trigger is pulled.  The striker is mounted on a rotating base and with each pull of the trigger the striker is cocked and simultaneously rotated to the next, unfired barrel.

The pistol uses a proprietary 4.5mm SPS pistol cartridge that has high penetrating power by replacing the typical bullet with a metal dart.  Underwater, conventional bullets are highly ineffective being inaccurate and limited to a very short range with a rapidly decreasing lethality.  The “bullet” has been replaced with a long 115mm (4.53 inches) dart weighing 12.8 grams (.452 oz.) made from mild steel that has a slightly flattened tip.  The fired projectile is kept stabilized by using a hydrodynamic cavity that is generated by the flattened point of the projectile that results in reduced drag, increased accuracy and lethality.  The dart has a longer range and more penetrating power than speargun spears.  The cartridge is a rimmed bottleneck case 40mm (1.575 in.) long and sealed against water.  The complete cartridge is 145mm (5.71 in.) long and weighs 17.5 grams (.617 oz.).

SADJ Editor-In-Chief Dan Shea as the test firer on the APS in free air. (Dr. Philip H. Dater)

The pistol is loaded with four cartridges at a time that are held together as a group by a clip in the same square arrangement as the barrels.  The pistol is broken open and the clip containing the four SPS cartridges are inserted into their respective chambers.  The action is closed and the pistol is ready to fire.  There is a single lever with three position options located on the left side of the frame at the top of the pistol grip, just behind the trigger, that controls the operation of the pistol.  The top position is for “Reloading” (barrel release), the middle position is “Safe” and the bottom position is “Fire.”

The pistol can be fired out of the water in the air environment.  Due to the pistol being smoothbore and firing a dart, the projectile is not stabilized at all and accuracy is greatly affected with an effective lethality range of 15-20 meters.  In an emergency, at very close range, it is still highly effective.

Range and lethality underwater changes the deeper the diver goes.  Depth reduces the range because the deeper one goes there is more water pressure that closes the hydrodynamic cavity sooner.  Once the projectile is no longer supercavitating, the hydrodynamic drag increases greatly resulting in the projectile becoming unstable.  The manufacturer claims that at a depth of 5 meters (16.4 ft.), the lethality range is 17 meters (55.17 ft.); at a depth of 20 meters (65.62 ft.), the lethality range is 11 meters (36.09 ft.) and at a depth of 40 meters (131.23 ft.), the lethality range is 6 meters (19.69 ft.).  Its stated lethal range is the range from which it can easily penetrate a padded underwater Prolon diving suit (flexible polyurethane padding) or a 5mm thick organic glass face mask.

The SPP-1M underwater pistol comes complete in a metal storage case that includes the pistol, ten cartridge clips, a holster that can attach to a diver’s belt, a device for loading the SPS cartridges into the clips, a special sling to carry the pistol and three combat ready metal quivers that each hold a set of four loaded SPS cartridges in their clips that the diver can attach to his dive belt.  Thus, the combat load is four rounds in the gun and twelve at the ready.

5.66mm APS Underwater Assault Rifle
The Tsniitochmash 5.66mm APS (Avtomat Podvodnyi Spetsialnyi – Special Underwater Assault Rifle) was produced at the requirement from the Soviet military to provide the underwater warrior with greater firepower and extended range than that of conventional underwater weapons and the SPP-1M underwater pistol.  Developed in the early 1970s, the APS was designed by a team headed by Vladimir Simonov, the nephew of Sergei Simonov, who designed the SKS carbine.

SPP-1M pistol as packed with loading tool (left), cleaning gear, as well as 10 ammunition clips. (Dan Shea)

As the basis of their design work, the designers began with the Kalashnikov rifle but discovered that although the AK would work under water, the bullets had virtually no effective range and were grossly inaccurate.  Back at the drawing boards, the designers came up with the APS design relying heavily on the lessons learned during the development of the SPP-1M underwater pistol.

The APS underwater assault rifle is smoothbore like the SPP-1M pistol firing a 120mm (4.72 in.) long 5.66mm dart projectile with a length-to-diameter ratio of approximately 21 to 1.  The MPS cartridge uses a standard 5.45×39 case sealed against water.  Using the same basic technology of the pistol, the long, thin projectiles are stabilized by water flowing along the sides and supercavitation to provide stability during travel rather than by spinning.

The rifle is gas operated and features a patented self-adjusting gas valve that allows the gun to be fired both under water and in atmosphere.  This self-adjusting gas valve was necessary because the energy required to operate the moving parts, particularly the bolt and operating rod, constantly change as the diver goes deeper and the water becomes more dense restricting the movement of the moving parts.  The rifle is select-fire for both single shot and full automatic with a single fire control selector switch on the rear of the left side of the receiver just above the pistol grip.  The fire control selector switch has three positions: forward for single shot, up for full automatic fire, and back for safety.  The rate of fire in full automatic is 600 rounds per minute in atmosphere but slows progressively as the gun goes deeper under water with the actual rate of fire depending upon the depth.  Information on the cyclic rate of fire at different depths was not available.  “Sources” note that while the APS can fire in atmosphere, the projectile does not effectively stabilize in the air, so its lethal range out of water is less than 100 meters and accuracy at that range is highly questionable – more than likely accurate out to about 50 meters.  Additionally, the expected service life of the APS when fired in atmosphere degrades severely.  Thus, it should be fired in atmosphere only in an emergency.  It was designed for underwater use, and it that environment, it is quite effective.

4.5mm cartridges for the SPP-1M pistol are packed 768 rounds per wooden chest, in two sealed cans of 384 rounds each. Each box contains 24 rounds, or six sets of four rounds per clip. In this picture, there are two of the clips to the left, and a set of four expended cartridges to the left of the four live cartridges in the center. (Dan Shea)

The sights of the rifle are rather crude with a non-adjustable open notch rear sight and a post front sight.  The rifle has a retractable shoulder stock made from steel wire that employs in a fashion similar to the U.S. M3 Greasegun.

The APS fires from an open bolt to allow the barrel to continually be filled with water, which is necessary for its reliable operation because of the type of projectile it fires.  The cocking handle and ejection port are both on the right side of the receiver.  The rifle feeds from a detachable polymer box magazine holding 26 rounds.  The shape of the box magazine is quite distinctive in that it is unusually deep (front to back) to accommodate the unusual shape of the MPS cartridge.  While the gun is relatively simple overall, the most complicated thing is the feed system where several parts had to be designed and installed to prevent double and even triple feed due to the extremely long projectiles.  The magazine release switch is located in front of the trigger guard and just behind the magazine.

Accessories for the APS underwater assault rifle include a canvas carrying case and a spare magazine pouch to wear on the diver’s belt.

The Russian government has publicly acknowledged only one instance of operational deployment.  In November 1989 at Malta, President George Bush and Russian Premier Mikail Gorbachev had meetings aboard a ship and the flotilla on which these two heads of state met were protected by a 16-man team of divers armed with the APS underwater assault rifle.

The rifle has been in active service with the Soviet and Russian special operations units for many years.  Tsniitochmash has been actively marketing the weapon since the 1990s and it is not known how many APS rifles have been sold on the international arms market to Western users, but it can be safely assumed that the rifle is in service outside Russia.

In today’s Global War on Terror, the threat is no longer just on land, on the sea and in the air.  Underwater threats to ships, ports, harbors, rivers, estuaries, offshore drilling operations and pipelines are just some of the targets that can no longer be ignored and are vulnerable to underwater attack.  The APS underwater assault rifle, and its little brother the SPP-1M underwater pistol, are clearly designed to meet these threats with use by special forces units, specialized law enforcement tactical teams and industrial security personnel – and they meet them quite well.

Because the guns are manufactured in Russia, they are not importable to the U.S. and Small Arms Defense Journal has not been able to evaluate or test fire either of the guns underwater – just the limited open air experience in Southwest Asia.

The story goes that a little over 25 years ago at a show, one of the prominent silencer dealers in the United States was approached by a curiosity seeker.  After looking around furtively, the seeker of knowledge asked the manufacturer, “Just how do these silencers work?”  After a moment’s hesitation, the answer came back in a whisper to suggest conspiracy: “Sound Catchers.  Like in a car muffler – you can get them at any auto supply store.”

There is a certain mystique as to how a structure attached to the end of a barrel can reduce the sound.  Hollywood has certainly added to this with their tiny appendages that change the sound of a firearm to a muted cough.  In the real world, about the only suppressors that can produce Hollywood results in Hollywood sizes are in weapons chambered for .22 LR and when using standard velocity (or target) subsonic ammunition.  The fundamental principles of suppression are not terribly complex, although the implementation may be.  For the purposes of basic understanding, this will look at only muzzle suppressors and the issue of the ballistic crack of supersonic bullets will be saved for a later time.

Any sudden release of pressure produces noise, and the higher the pressure (and shorter time interval for its release), the louder will be the sound generated.  This is particularly true in a firearm.  The pressures propelling the projectile are generated by the explosive burning of the powder, producing large volumes of gas.  The gas under pressure has to not only overcome the friction of shoving a bullet through a too-small tube, but to accurately accelerate the bullet to a point where it can impart energy to its target.  The actual pressure generated depends on the caliber, the amount of powder, the burn rate, and the barrel length.  At the instant the bullet exits the rifling, which we refer to as “uncorking,” there is a very sudden release of this high temperature, high pressure propelling gas making a significant sound.

On the left are samples of the so-called M-baffle, which consists simply of a cone (the baffle) with an integral spacer. In cross section (top), it resembles the letter “M.” On the right are representative samples of the K-baffle. These appear to be a relatively flat baffle with an integral conical spacer and various jets to create turbulence. In cross-section, they resemble the letter “K.” (Phillip Dater, MD)

The object of a suppressor is to reduce the pressure of the propelling gas while it is still within the weapon system and to delay its exit.  Assuming complete combustion of the powder, the propelling gases will have a known mass of the gas (or known number of molecules).  Just before the bullet uncorks from the barrel, the pressure in the bore will vary from 4-5,000 psi up to 30,000 psi or so, depending on the caliber, amount of propelling powder, and barrel length (volume).  To reduce the sound, one has to reduce the pressure.  This gets into basic high-school physics.

One way to reduce the pressure is to increase the volume, with more volume being better.  The disadvantage is simply one of practicality.  If it were possible to reduce the bore pressure to the normal atmospheric pressure of 14.7 psi, there would be no noise.  The necessary suppressor size is inconceivable.  Instead, if the suppressor’s free internal volume is about 20 times the internal volume of the bore, the pressure reduction will result in sound level reduction in the vicinity of 13 or 14 dB assuming no reduction in temperature of the gases.  Each halving of the actual pressure reduces the sound level by 3 dB.

The second method of reducing pressure is to reduce the temperature of the propelling gases.  This is one of the two functions of the baffles, which present a large surface area to absorb heat.  Metals with a high specific heat will absorb heat faster (and conduct it faster).  The heat then dissipates from the suppressor by conduction to the outside shell and from there by convection and radiation.

Suppressors for handguns present a unique problem, depending on caliber.  There are definite size limits due to cycling, sighting, and concealability issues as well as having a physically large suppressor making the weapon unwieldy.  Introduction of a liquid or gel (an ablative agent) will absorb heat rapidly as the liquid or gel changes phase (boils).  Converting water to steam absorbs far more heat than simply heating the water (it requires 1 calorie to raise 1 gram of water 1°C, but 540 calories to convert 1 gram of water at 100°C to steam).  Ablative agents lend themselves well to pistol caliber suppressors, but should never be intentionally used in centerfire rifle suppressors.

These are simple stamped baffles formed into a cup, providing an integral spacer. On either side of the center hole, scoops are formed to generate turbulence. They are stamped from thin steel. While the spacing may not have been optimal, they are reputed to be quite effective, especially on a .22 rifle. Similar baffles have appeared in other European manufacturer’s rimfire suppressors (including some Maxims, which may have been the origin). (Phillip Dater, MD)

In addition to reducing pressure, the sound level can be reduced by spreading out the timeline of gas exit from the suppressor itself through generation of internal turbulence, disrupting the gas flow.  As an example, a balloon makes a lot more noise if the pressure is suddenly released by rupturing the balloon rather than simple opening the valve and allowing the pressure to dissipate slowly.

Baffles, even the simplistic ones, do trap some of the gas exiting behind the bullet, delaying exit and allowing slightly more time for heat absorption.  Considering that the gas following the bullet is traveling forward and expanding, almost any partition with a small hole for bullet passage will delay the gas exit, disrupt flow, and cause turbulence.

Simple conical baffles work amazingly well in moderate volume suppressors.  Various surface irregularities and jetting cuts will increase turbulence and their effectiveness.  With simple baffles, proper design of the spacer can contribute to gas trapping.  There are more modern baffle designs that are engineered to create maximum turbulence and trapping, but many of these have limitations.  Complicating the design process is the user’s requirement for smaller, lighter, more efficient units.

There is no single baffle type that is ideal for every possible weapon/cartridge combination, and there is no magic formula to determine the best number, spacing, and design of baffles.  The general principles are fairly basic, and most are implementations of the General Gas Law.  Hiram Percy Maxim patented his first suppressor 100 years ago, and continued fascination with silencers has inspired numerous and often innovative designs ever since.

The Soviet Union and its current iteration, Russia, have long been known for innovative weapon development; only some of which has seen the light of day in the Western world.  There is a long history of effectively “keeping the lid” on new weapons until someone on the other side runs into them in the field and reports on them.  Rumors of a new cartridge and both a submachine gun and silenced sniper rifle utilizing this ammunition have leaked into the general Western military communities for many years, some have been on display at shows, and there have been several Western military personnel and firearms writers who have studied these and written some information on them.

None of these unique weapons have been seen by the general military population other than in isolated news clips, especially from the recent unpleasantness in the former Soviet State of Georgia.  When Russian military forces crossed into the northern provinces of Georgia and swiftly cut them off from the south, much of the world recoiled in horror and voiced platitudes about how the Russians must stop this assault.  Within a short period, the Russian military forces brilliantly transitioned into the “Peace-keepers” in the region, and photos leaked out to the Western press and intel communities showing strange Dragunov-looking weapons that were clearly too short to be in 7.62x54R caliber, and also very clearly integrally suppressed.  Pandemonium ensued as all resources were called upon to ID this weapon and the threat it represents.

There have been some excellent but isolated references on the 9x39mm ammunition and firearms, and Internet resources are limited of course to the writer’s experience with the weapons – usually non-existent other than in computer games.  Real time, take-it-apart, pull the trigger, hands-on testing has been very rare and certainly not widely reported.  Charles Cutshaw’s excellent book The New World of Russian Small Arms and Ammo (ISBN-10: 0873649931) does an excellent job and should be on everyone’s bookshelf, but his 1990s treatise is limited to Marshall Goldberg’s line drawings for illustration and he was not allowed to disassemble the weapons.

The MA “VIKHR” submachine gun is in reality a compact assault rifle since the 9x39mm cartridge it fires maintains its energy beyond 400 meters, even with this barrel length. It is not designed to be fired at distances anywhere near that, but the little powerhouse has that capability built into the ammunition. The VIKHR is designed for reliable full automatic fire, it is compact, can use either 10- or 20-round magazines but the 20-round is designed for this weapon. Note the over-folding sheet metal stock, the hinged top cover, and the tubular striker firing system. At only 2 kg, (4.4 lbs) the VIKHR is a briefcase sized fist full of power.

Two of the Small Arms Defense Journal writers were granted the opportunity to examine in detail both the weapons and the ammunition in the field at a discreet location in Southwest Asia.  In addition to extensive disassembly and photography, we were also given the opportunity to test fire both the VSS silenced sniper rifle and the VIKHR submachine gun with our meters ready to conduct scientific testing.  We wanted to be as thorough as possible with this unique opportunity.

There are some myths and assumptions made that we will try to gently correct, or at least provide another point of view on.  The first being that the MA “VIKHR” (Whirlwind) submachine gun is the same as the VSS “Vintorez” (Thread-cutter) with the exception of the buttstock and suppressor.  This is not true as the receivers are similar, and some parts will interchange, but they are not the same receiver with one simply being suppressed.

As is their standard procedure, the Russians had several factories competing to manufacture the 9x39mm weapon systems they required.  The end product offerings are as follows:

TsNIITochmash:

• MA “VIKHR” (Whirlwind) unsuppressed miniature assault rifle.
• AS “VAL” (Rampart) based on VIKHR, side folding stock, not a “Take-Down”.
• VSS “Vintorez” (Thread-cutter) based on VIKHR, removable stock, “Take-Down”.

Tula KBP:

• A-91 offered in a number of calibers, can operate with or without the suppressor.
• VSK-94 sniper model of the A-91, in a case, could have wood (early) or polymer stock and fire without suppressor installed.

Our concern today is with two of the offerings from TsNIITochmash, the VIKHR and the VSS.

The VSS “Vintorez” is based on the VIKHR, but the receiver has notable differences and they do not interchange. The VSS is the weapon most readily identified in Georgia, and the system will not operate for more than one round with the suppressor removed as it was not intended to be fired with the suppressor removed. The exposure of the ported barrel alone would be a dissuader. The VSS weighs 2.6 kg (5.7 lbs) with suppressor, optic, and empty magazine. It can be broken down into component parts – suppressor, receiver, optic, buttstock, magazine, and packed into a very small space. It is quick to reassemble. The system does not use a standard hammer utilizing the tubular striker instead, and has very little in common with a Kalashnikov other than the appearance of some controls, and the fact that it is gas operated.

The AS-VAL is .1 kg lighter (.2 lb) than the VSS, and is about one inch shorter, but it can not be broken down for discreet use. The visual identifying signature of the VAL is the side folding stock. Other than that, it has very similar characteristics to the VSS.

SADJ did a complete comparative photo series, but space does not permit the publishing here. These two photos should show the similarities between the VSS and the VIKHR parts, and how minor differences make most parts incompatible. Aside from the VIKHR lacking an optical mounting rail, the receiver has a different shape at the rear. This is partly to facilitate the different types of selector used: the VIKHR is a push-through type while the VSS has a more tactile sniper-friendly lever behind the trigger. The interior fire control is basically the same other than the physical block differences. At first glance, the bolt carriers look identical, and they are very close, except that the VSS uses a charging handle on the bolt carrier while the VIKHR uses a forward ambidextrous system with a push-rod.

The VSS has been seen issued with the 1LH51 second generation night vision device (not shown), among other optics, but the PSO-1 variant shown here is standard with the IM2-1 marking. Note the “Hammer & Sickle” USSR marking. While the optic gives the appearance of being the same as a Dragunov scope, the stadia lines are different - estimating range only to 400 meters, and no adjusting Point of Aim chevrons. The optic has an illuminated reticle with remote power capability.

LEFT: The VSS is shipped in its own wooden chest, along with the following accessories: (Top left) Accessory/cleaning kit pouch. (Center) Oil bottle, WTH*, cleaning rod, screwdriver tool, remote battery cable and sling. (Right) Weapon carrying case suitable for weapon in tear-down state. (Lower left): Optic cover. Note: WTH* is a “What the Heck” because we have no idea what this round, scalloped sheet metal piece is or does, but it was in the chest, so is included here. RIGHT: The magazine for the VSS, VAL, and VIKHR all interchange. They are made of polymer with a standard style metal spring and are either 20 or 10 rounds. The 10-round magazine is basically for the VSS to lower the profile. The 20 round is for the VIKHR, but it is more concealable with a 10 round for carry. Doctrine appears to have evolved that operators use whichever they want.

The barrel of the VSS is approximately 12 cm longer that the Vikhr barrel, but the last 9 cm features barrel porting. The porting consists of six rows of nine ports spiraling along the rifling grooves. The twist rate is 1:210 mm (1:8.3 inches). Each port is approximately 2 mm diameter. The non-ported length of the VSS barrel is the same as the barrel in the Vikhr, which would lead one to believe that the muzzle velocities will be comparable. On the range we subsequently showed this to be the case. Visible in the second picture are the ports on the VSS barrel, as well as the large thread bushing that matches the interrupted threads on the trunnion for fast rotation of the suppressor on or off. This part is readily removable and should be cleaned, but if lost the suppressor will not mount or align properly, and will certainly put the weapon out of commission.

The silencer is exceptionally simple, especially when compared to Western designs, and bears more than a passing resemblance to the suppressor on the Soviet PB silenced Makarov. The entire silencer is unscrewed from the firearm after depressing a small button latch on the front of the frame of the firearm. This reveals the ported portion of the barrel. As shown in the accompanying photos, the stack is made entirely of spot welded sheet metal. The three baffles are punched and bent from a strip of 0.8 mm steel and are in the form of slanted “washer-type” oval baffles with the first and third slanting approximately 30 degrees (from perpendicular) one way and the middle slanted opposite. The strips, functioning as tabs, are spot welded to two longitudinal strips with a flat washer-baffle spot welded at each end. Reassembly consists of reversing disassembly.

It is easy to spot older, long-time shooters.  These are the guys who can’t understand a word you are saying while regaling you with tales of the Great Hunts in exotic locations, competitions won, or battles bravely fought.  Some will use hearing aids, which are second choice to natural hearing.  Others will be able to understand you if they can see your lips move and there is little ambient noise.  There are those who combine both categories.  What has happened to these guys?

Simply put, they have had significant hearing damage due to chronic exposure to loud noises.  The ear is a sensitive mechanism designed to translate rapidly changing air pressure variations we refer to as sound into nerve impulses that the brain can use for communication.  The hearing mechanism is more than just the floppy appendage on the side of the head (the pinna, which most animals can change the shape of to concentrate sound).  In addition, the external ear consists of the ear canal and the ear drum (tympanic membrane).  Considering only the hearing mechanism (not balance), the middle ear contains a series of three tiny bones connecting the tympanic membrane with a similar structure (the oval window), that forms the boundary of the inner ear.  These bones, the smallest in the body, form an ingenious mechanical sound amplifier.  With aging, their joints can develop arthritis, decreasing their ability to transmit sound.  The inner ear consists of the cochlea, which is a fluid-filled spiral structure having small sensory hairs.  These hair-like structures resonate at different frequencies, stimulating the underlying nerve endings, sending the sound bits to the brain.  These sensors are fairly fragile, and loud noises will over-stimulate them, leading to early failure.  The shorter ones (representing the higher frequencies) are the closest to the oval window and are more easily damaged.  There is some attenuation of the sound level in the liquid in the cochlea as the sound wave traverses to the far end, which explains why high frequency hearing is damaged sooner than the lower frequencies.   Once failed, the associated frequency can no longer be perceived.  We do get a little warning that the hearing mechanism is being damaged by “ringing” in the ears (tinnitus).  Early on, this is transient and indicates an insult to the hearing mechanism.

There are two basic mechanisms of hearing damage.  Sustained loud noises cause an inflammatory reaction in the tissues supporting these sensory hairs.  With time and continued inflammation, these sensory hairs eventually fall out.  The second mechanism is a shock from a really loud sound (over 140 dB), which mechanically shears some of the sensory hairs.  An analogy would be tooth loss from either gum disease (chronic inflammation) or acute trauma, such as a blow to the mouth.  Either way, there is permanent damage.

Drawing of the ear and hearing mechanism showing the components of the ear. The outer ear consists of the pinna, external auditory canal, and tympanic membrane, which is the boundary with the middle ear. The middle ear consists of the three tiny bones (maleolus, incus, and stapes) that form a mechanical amplifier and ends with the oval window, which is the boundary of the inner ear. The inner ear consists of the cochlea (hearing mechanism starting with the oval window) and the semi-circular canals relating to the balance mechanism. (Drawing courtesy of Brüel & Kjaer)

Other than totally avoiding loud sounds, there are several protective measures available.  The first is directly protecting the hearing mechanism with mechanical barriers of some sort.  These generally consist of either insulated earmuffs or insulating plugs that are inserted into the ear canal.  Of the two, muffs provide far superior protection.  The plugs in the ear canal do an acceptable job of reducing the sound to the tympanic membrane and the normal hearing path.  However, sounds can be (and are) transmitted directly to the cochlea through the bones of the base of the skull.  Bone conduction is the way we normally hear our own voices when we talk and is the reason why our voice sounds so strange to us from a tape recorder where we hear it through the normal pathway.

In-the-ear hearing protection, while better than nothing, is not as effective as muff-type protectors, which reduce sound levels by 28+ dB (including the bone conduction pathway).  Until about 30 years ago, few people used hearing protection when shooting, and even today hunters rarely use protection in the field.  Because of the logarithmic nature of sound measurements, using both muffs and plugs adds at best 3 dB to the reduction of the muffs alone.

The second protective measure is suppressing the sound at the source.  The past several years have seen a proliferation of new firearm suppressor manufacturers, some of whom offer highly affordable suppressors.  There are those who preach that actual reduction (or absolute sound level) measurements are not meaningful and preach that perceived loudness is all that counts.  Nothing could be further from the truth, and believing this fallacy can lead to significant hearing loss.  Perceived loudness depends on three factors: the frequency of the sound itself, the duration of the sound pulse, and the observer’s existing degree of hearing loss.  Significant hearing damage can be sustained from gunfire, regardless of how quiet something sounds. In addition, hearing damage is caused by more than just gunfire, and the dose is cumulative.  Other sources are industrial noise, wind noise (motorcycles, driving with the window open), entertainment, etc.

OSHA (Occupational Safety and Health Administration) requirements for industrial exposure (sound levels in the workplace) are that hearing protection must be used if the sustained sound level is above 85 dB, or at least the sound must be reduced to below that level at the ear.  This assumes an exposure of 85 dB day in and day out, 40 hours a week for the entire work year.  This standard also assumes that the worker is exposed to no sounds over this level.  In theory, if there are no sounds over this level, there will be no hearing damage.

The family of equal loudness contours show the difference in sensitivity of the ear to sound levels at differing frequencies. The graphs show the actual sound level necessary to have the same apparent loudness as a 1 kHz tone at a given level. As an example, a 50 hZ tone will need to be approximately 15 dB louder than a 70 dB tone at 1 kHz to appear to have the same “loudness.” (Graph redrawn from Brüel & Kjaer)

Besides at work, there are a lot of other sources of potential sound damage to hearing.  Rock concerts come to mind, and there are a number of young fans of rock concerts sustaining significant hearing damage at concerts where the sound levels in the front row (where one can really “feel” the music) may be in excess of 130 dB.  The simple portable CD or cassette player with headphones can (and often is) cranked up to the point where the sound level delivered to the ear is dangerous.  As an aside, if you can hear the music when your child is wearing the headphones, it is too loud.  Other sources in the day-to-day environment include wind noise while riding a motorcycle, driving a convertible, or driving with a window open.

Other than the portable cassette/CD player, the dose level may not be sustained long enough to damage hearing if there are no other significant sources of dangerous noise in the workplace or environment.  The problem is that the dose to cause hearing damage is cumulative.  The maximum OSHA permitted level is for 85 dB for 2,080 hours in the year (the number of hours assuming a 40 hour work week).  The problem comes in when one considers the nature of sound.  Each 3 dB increase in sound level is an actual doubling of the absolute sound pressure level.  In other words, if 85 dB is the maximum acceptable dose for sound in the 2,080 hour year, if the exposure is raised to only 88 dB (a 3 dB increase), the subject will receive the same equivalent dose of sound in half the time (or 1,040 hours).  With each 3 dB increase in sound level, the acceptable time is cut in half.  For the 130 dB rock concert, hearing damage will start at about 17 hours, assuming no other damaging sound levels.  And this assumes the rock concert is only 130 dB.  Many rock musicians are prematurely deaf by age 30.

Firearm noises are another source of noise pollution, and they are unique due to their extremely short duration.  As such they may not sound terribly loud, and this perceived sound varies with the caliber and circumstances of firing.  An air rifle is thought to be essentially silent, but it may well generate a sound pressure level of 110-130 dB, depending on design, type, and caliber.  However, because the sound is of such short duration, it is almost impossible to come up to a dose equivalent to the OSHA standard of 85 dB over a work year of 2,080 hours.

As the firearm sound gets louder, however, the number of permissible exposures drops to the point where a 160 dB 9mm handgun will give enough exposure to reach the OSHA yearly equivalent in only 7 rounds.  This becomes a real concern, especially to the hunter (either shotgun or rifle) who doesn’t wish to wear hearing protection in the field.

The simplest method of measuring the sound reduction of muff-type protectors is to simply clamp a set around the microphone after making readings from a non-suppressed pistol. In this case, we measured a common set of Peltor electronic hearing protectors using an FN-57 pistol (163 dB) as the sound source. The reduction measured with the protectors rubber-banded around the microphone was over 32 dB (electronics off). We obtained the same reading (within 1/10 dB) with the electronics on and the volume set at maximum. With electronic hearing protectors, the batteries should be changed at regular intervals rather than waiting for battery exhaustion. (Photo by author)

Both OSHA and MIL-STD-1474D state that hearing protection MUST be used if the sound pressure levels of firearm noises are 140 dB or greater.  This is specified in Requirement 4 of MIL-STD-1474D.  While it is true that a sound level of, for example, 145 dB is not nearly as harmful as one of 160 dB, exposure will still cause damage.  Once the damage has occurred, it is forever and hearing will not come back.

MIL-STD-1474D has specific requirements for metering equipment, which eliminates the Radio Shack types of meters (and, interestingly, many of the current Type-2 field-portable units).  This was discussed in detail in the Spring issue of SADJ.

The standard location for sound measurement is 1 meter to the left of the muzzle at 90 degrees to the bore axis and 1.6 meters above grass.  This is the location that most of us use in the United States.  A second location, which is used to simulate the shooter’s ear, is 1 meter from the muzzle at an angle back from the bore axis of 15 degrees.  Experience has shown that this second location is generally 2-4 dB less than the one at 90 degrees.  Measuring actually at the shooter’s ear location is somewhat inconsistent because of shadowing by portions of the shooter’s head.  The bottom line is that if a sound suppressor does not meet the US standards specified by OSHA and MIL-STD-1474D, then it can cause hearing damage.

Although some suppressors may exhibit frequency shifting to where a significant portion of the sound level is at the upper limits (or perhaps even beyond) the range of human hearing, if the level is above 140 dB, hearing damage still can occur.  It is necessary to remember that each individual has a different hearing loss, and what may be beyond the hearing of an elderly industrial worker may well be within the range of hearing of a young person.

While sound measurements have been used as advertising hype, the important thing is that they indicate that the suppressor reduces the sound below the US standard of 140 dB.  The degree of reduction is not really as important as the absolute sound pressure level.

In the interest of not damaging the hearing of customers, all manufacturers should have their suppressors measured by someone knowledgeable in the field of gunfire sound measurement and who has in his possession equipment that meets the MIL-STD-1474D requirements for these measurements.

Firearm related hearing damage is a serious concern today and will continue to be a source of liability.  The largest percentage of claims in the Veterans Administration hospital is related to hearing damage and averages over $4,000 per year per veteran.  Hearing damage claims are starting to show up in law enforcement agencies, and noise pollution from law enforcement training is the source of numerous complaints in the general population.

In my opinion, it is a disservice with today’s knowledge to promote a suppressor that, while far better than nothing, still has a sound level that exceeds US standards for safety without hearing protection.  In the field of medicine, this would be considered contributory negligence, and in today’s litigious climate, it may well result in legal action against a manufacturer.

Little is known in the Western world of the Soviet silent pistols utilizing special silent ammunition. The package consists of the model PSS (Pistolet Sptsialnyj Samozaryadnyj) silent pistol and the special SP-4 captive piston ammunition. The testing performed consisted of evaluation of the basic handgun, the special ammunition, sound pressure measurements, muzzle velocity, and terminal performance.

In this report, a mixture of English and metric parameters are used. All linear and circular dimensions are metric (millimeters). All weights are grains simply because the convention we use are to weigh bullets and powder in grains. Velocities are English and in feet/second because those are the units of our chronograph. Sound levels are decibels (reference zero dB as 20 micropascals). Temperatures are dual units (F/C) and barometric pressure is in millimeters of mercury.

PSS Pistol
This finely crafted semiautomatic handgun is a magazine-fed weapon utilizing a single stack magazine that holds 6 rounds and a last-round slide hold-open device. The pistol is well finished, and all parts carry the pistol’s serial number. With the finish and serialization of all parts, it appears that during factory assembly on all examples seen by this author, all parts were hand fitted. It is issued with a brief instruction manual (in Russian) and a somewhat flimsy tan leather shoulder holster.

The PSS and its specialty SP-4 ammunition were specifically designed for elimination of live targets without risking discovery of the operator. Unlike predecessor captive piston handguns, the PSS is capable of semiautomatic fire. Although there is a definite advantage to rapid follow-up shots, the disadvantage for the covert operator is that it ejects (and leaves) spent cases at the scene. These spent cases are exceptionally distinctive, and almost anyone with an even passing familiarity with this weapon will be able to identify its use from the spent cases.

The author has just fired the PSS, and the slide has locked back on an empty magazine. (Photo by Dan Shea)

It is not obvious where either the PSS pistol or its SP-4 ammunition is manufactured, but other clandestine special purpose weapons have been built at Tula Arsenal, and it is suspected that Tula may well be the origin of the PSS. The weapon carries absolutely no markings other than a low four digit serial number. The PSS was developed for special personnel of the Soviet KGB and for elite elements of the Spetsnaz of the Soviet Army and was introduced around 1983. It is currently used by many elite Russian anti-terrorist teams. It is far more compact and has a quieter action than the more common Soviet PB (silenced Makarov) and Chinese Type 67 silenced pistols.

While the PSS resembles a somewhat large conventional blowback pistol, it is definitely unique. It will accept no ammunition other than the SP-4 silent cartridge. Other than the ammunition used, the most obvious is it has a two-part barrel. The separate distal rifled part is fixed to the frame. The breech portion (consisting of the chamber) is allowed to recoil inside the frame for a short length against its own return spring. This increases the mass of the moving parts at the initial stages of recoil, and also slows the slide on its final stages of movement, resulting in dampening the sound of the slide hitting the stop on the end of the recoil cycle.  The slide return main spring is housed in the slide, above the barrel, and the spring guide rod is part of the slide retaining assembly. The PSS firing mechanism is single/double action with open hammer and slide-mounted safety/decocker of conventional Makarov design. Sights are fixed.

We were granted the opportunity to extensively examine (including total disassembly), photograph, and fire the PSS at a discrete Southwest Asian military arsenal. To the best of our knowledge, there are no examples of this weapon in the United States at the time of writing. There are known to be several examples in the United Kingdom. Distribution through the former Soviet satellite countries is unknown but presumed to have been issued to clandestine units in these countries. Total production is unknown.

Because all products of combustion are contained within the spent cartridge, there is no powder fouling or possibility of corrosion from powder residues. Interestingly, this also means that the operator’s hands will not be contaminated with powder residues, which may be of interest in the forensic community. Maintenance consists simply of light oiling of the weapon and wiping the external surfaces with an oily rag.

We do not have available to us at this time a factory breakdown of parts. Because of this, the names we use are based on apparent function and similarity to parts in more conventional pistols. Field stripping for basic cleaning and lubrication can be accomplished with no tools other than a section of cleaning rod. Before starting, the magazine must be removed and the hammer cocked. Although not photo detailed, the accompanying pictures show a slot cut in the front assembly locking piece, which also appears to function as the slide stop. This engages a cut in the front of the recoil spring, limiting rearward motion of the slide. To field strip, a section of cleaning rod is inserted into the recess in the top rear portion of the slide, and the recoil spring guide rod is pushed forward until the locking piece/slide stop clears the front end of the barrel. At this point, the locking piece can be slid upward to disengage from the front of the recoil spring guide rod. The slide then removes to the rear, but the forward end of the slide must be disengaged upward from the rear of the floating chamber, before it can be fully removed. Re-assembly is the reverse.

While we completely disassembled the PSS, we can most definitely recommend that the user never do more than simply field-strip the weapon for maintenance. Total disassembly is quite difficult and is by no means intuitive. It is also totally unnecessary. Re-assembly is even more difficult. We speak from experience.

The front assembly locking-piece/slide-stop has been removed from the barrel and the groove in the front of the recoil spring guide rod. This is the first step in field stripping. At this point, the slide can be removed to the rear. (Photo by author)

SP-4 Ammunition
Captive piston ammunition is a unique, special-purpose cartridge that is designed to be intrinsically quiet as compared to conventional ammunition. Essentially, the SP-4 silent cartridge consists of a reinforced steel cartridge case containing a small powder charge enclosed in a cup-like piston in the rear of the case. This piston rests against the base of the projectile; a cylindrical bullet made of mild steel and fitted with a brass driving band at the front. The SP-4 cartridge was adopted by the Soviet KGB and Spetsnaz units in 1983 along with the PSS host pistol.

The ammunition is issued in plain white boxes of twelve cartridges each. The only markings on the box are what appear to be a lot number. This is enough to charge both magazines. There are 20 boxes in a hermetically sealed SPAM tin and two tins in a wooden crate. The tins are marked with the cartridge designation, lot number (that correlates with the number on the boxes), and some other markings of unknown meaning.

Many prior designs, such as the SP-3 and PZAM silent cartridges, utilized 2-part (or 2-stage) piston that extended beyond the end of the cartridge case. While this permitted greater acceleration of the projectile, it also was incompatible with cycling in a semiautomatic host weapon. These older silent cartridges from the late 1970s and early 1980s utilized the standard 7.62mm M43 projectile loaded in the AK 7.62×39 rifle cartridge.

When fired, the primer ignites the powder charge, and the rapidly expanding gases drive the piston forward at a high acceleration. The piston, in return, drives the projectile. When the driving piston reaches a shoulder at the end of the case, the piston is somewhat violently stopped by a shoulder on the front of the cartridge case. Having been accelerated, the projectile engages rifling in the short barrel and exits the weapon toward its target. The high pressure propelling gases are contained (and trapped) in the spent cartridge and gradually leak out over a period of several weeks. Because the propelling gases are not released into the atmosphere, there is very little sound generated.

Suppressor Sound Measurement

Sound is defined as air in motion, which produces pressure variations that the human ear can detect.  It is a form of overpressure, and how it is interpreted depends on a variety of factors, including frequency, duration, intensity (amplitude), and variations in the observer’s hearing acuity.  The subject of this column is related to firearm sounds.

Firearm sounds are produced by the sudden release of exceptionally high pressure gases from the barrel at the instant of the projectile exiting the muzzle end (uncorking).  Depending on the cartridge and barrel length, these uncorking pressures can vary from around 5,000 to 62,000 pounds per square inch (psi).  The pressure drop is almost instantaneous, occurring in considerably less than 50 milliseconds (0.050 seconds).

Sound levels are usually expressed as decibels (dB), which is simply the logarithm (base 10) of the ratio of the actual pressure referenced to the lowest pressure that a human can perceive (defined as 0 dB).  The threshold of human hearing is approximately 5 billion times less than normal atmospheric pressure while a firearm’s sound may be over 4,000 times greater than atmospheric pressure.  A logarithmic scale is used because actual pressure ratios become unwieldy numbers.  A 3 dB difference is a doubling (or halving) of the actual pressure level and is a change that most observers can identify.

Estimation of loudness has varied from simple subjective observation (standing at a range and listening to a firearm) to use of meters designed for home balancing stereos to precision meters for measuring continuous industrial noise levels to meters with specific characteristics for firearm sounds.  With the exception of the latter, the results are inaccurate and can lead to a false sense of hearing safety.

Bruel & Kjaer 2209 sound meter with B&K 4136 pressure microphone.

Firearm sounds are of exceptionally short duration.  Hearing perception varies primarily with the frequency of the sound as well as the duration and the distance from the sound source.  Short duration isolated impulse sounds subjectively are not nearly as loud as a continuous sound.  For example, a jack-hammer and a pump air pistol have similar peak sound levels.  In addition, low frequency sounds subjectively are louder than those between 3 and 5 kHz, and the further one is from the sound source, the less intense the sound appears to be.

Numerous attempts have been made to create a standard for firearm sound measurements.  OSHA standards are most applicable to relatively continuous industrial noise.  The current most comprehensive standard for military noise limits and measurement techniques is Mil-Std-1474D, and the limits are generally more stringent than the OSHA standards.  While Mil-Std-1474D addresses noise risks from a variety of military equipment (including cannons and naval guns), it has a section specifically addressing small arms, which is the only section that will be addressed in this column.

A little over 30 years ago, there were no accurate standards for measuring small arms sound levels, both non-suppressed and suppressed.  Measurement criteria in the Frankford Arsenal Report 1896 (August 1968) were an attempt to quantify sound levels with the tools available at the time.  In light of today’s equipment, these results were grossly inaccurate.  Three early pioneers in sound suppressor development created a method of quantifying suppressed firearm sounds.  They were C. Reed Knight, Jr., Mickey Finn of Qual-A-Tech, and Don Walsh of Larand.  Their protocol, with only few modifications, morphed into the small arms measurement portion of Mil-Std-1474D, Requirement 4.

The meter used must meet the requirements for a Type 1 precision sound meter and be capable of measuring the highest sound peak generated by firearms (usually in the vicinity of 170 dB, although some firearms will be louder).  In addition, the rise time of the entire system (meter and microphone) must be 20 microseconds or less.  This is simply because the initial (and highest) peak has a duration only slightly greater than 20 microseconds, and any shorter rise time may well miss the peak.  30 years ago, there was only one field-portable instrument that met those criteria: the B&K 2209 with the B&K 4136 1/4 inch pressure microphone.  Subsequently, the Larson-Davis 800B meter was introduced, meeting the same requirements.  Sadly, both of these meters are out of production and their more modern replacement field portable meters have a far greater rise time, limiting their usefulness for measuring the shorter duration peaks of suppressed firearms.

Authorities in the measurement of overpressure associated with explosions suggest that sound pressure measuring systems featuring a 20 microsecond rise time (such as the B&K 2209 or LDL 800B) will miss some of the initial peak.  Although this may be true, the bias is probably small (in the order of one or two decibels).  In spite of this small bias, the standard of the firearms suppressor industry, NATO, and the US military is to utilize instrumentation with a 20-microsecond rise time.