The Sounds of Silence: Part 4

Pressure testing the entrance chamber of a 1.38 inch diameter 5.56mm suppressor on a 10.4 inch barrel. Pressure measurements of 2,998 psi calculated an acceptable safety factor at ambient air temperature and a marginal safety factor after 100 rounds fully automatic fire. The significantly lower chamber pressure on a 14.5 inch barrel (the intended platform for this suppressor) resulted in acceptable safety factors both cold and following fully automatic fire.

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.