At this time in 1967, a congressional sub-committee headed by Senator Richard Ichord convened to investigate the AR-15/M16 and all of the issues surrounding the procurement, rights to the technical data package, chain of events to milestone decisions and malfunctioning. Colt engineers went to work making a version of their AR-15/M16 rifle that would make use of the (perceived) more reliable long stroke piston of the AK47.

The first external piston version of the M16 series rifle is the Colt Model 703. Careful examination shows that Colt engineers knew in the 1960s that to correctly make an external piston operated version of the rifle a completely new upper receiver was required and the rifle had to be designed as an external piston rifle and not converted.

The Colt Model 703 was the first external piston operated rifle of the platform. For all intents and purposes the Model 703 left the ergonomics of the AR-15 and stuck an upper on it designed around a piston operated rifle. The lower receiver remained the same as the new upper receiver could retrofit any stock lower receiver. The prototype had the Foster Sturtevant designed four-position selector offering the shooter safe, semi-auto, auto and 2 or 3 round burst.

The Colt engineers back then realized the best way to make an external piston operated rifle was to redesign the upper receiver, not to take the current one designed for direct gas impingement and turn it into a host for an operating system it was not designed for. The bolt carrier has the long stroke operating rod/piston pinned to the top of the bolt carrier. The bolt carrier has two longitude grooves on the side that fit into two slots in the upper receiver acting as guides to keep the bolt carrier group moving straight back and forth in the receiver (no carrier tilt).

The gas system utilizes a gas regulator allowing the shooter to tune the rifle for any combination of ammunition type or environmental condition. Self-regulating gas systems are more finicky due to extremes of ammunition types, propellants, as well as weather and atmospheric conditions. Allowing this all to be regulated is an excellent solution. The problem is if an untrained user messes around with the regulator not knowing how to adjust it properly, the rifle may not function, and may beat itself up from over gassing or function unreliably. It has always been a double edged sword whether to permit the soldier access to adjust gas or take a self-regulating system and try to control the ammunition that goes into it.

Then next generation Colt model LE1020 is based on the Type-C SCAR rifle but many refinements were added built on comments/criticisms of the Type C rifle. This would include making the gas system easily removed for maintenance and cleaning, elimination of the UCT finish and a simpler way to attach the carrier key to the bolt carrier, which did not require machining. Later, the steel insert would be added to the rear of the cam pin, which prevented damage to the receiver seen in all other external piston AR-series rifles.

Colt issued a report in May of 1969 titled Colt’s Automatic Rifle Model 703. The lead off into the report stated, “It is Colt’s belief that Colt’s Model 703 will demonstrate better performance and superior durability when compared to the M16/M16A1 rifle.” Nobody can be sure, all of the engineers who worked on this project are gone, but evidence points to the fact Colt was both involved in the investigation of what was in fact going wrong with the AR-15/M16 rifles in Vietnam but they also wanted to be ready with a solution if the problems pointed to the operating system of the AR-15/M16. It appears as though the Model 703 was a proactive step on Colt’s part.

According to the report, “Prototype Number 1 features a closed, adjustable gas system which is completely housed in the front sight assemble. This allows the user to optimize his weapon’s efficiency according to conditions dictated by climate or ammunition. No gas deposits are permitted to enter the breech mechanism and the result is a cleaner operating weapon. In addition, a larger extractor, a failure-free extractor spring and increased dwell time before unlocking have been incorporated.” The report also stated a list of features of the Model 703:

Better extraction through delayed unlocking and stronger extractor.
Top and side sling capability.
Adjustable gas system.
Burst control (2 or 3 rounds).
Buttstock stowage of cleaning equipment.
Simplified handguard disassembly.

Field stripped Colt Model 703. Note how the bolt carrier has rails on the side that fit in a groove inside of the upper receiver. This prevents carrier tilt. Also notice the long stroke piston operating rod connects right to the top of the bolt carrier.

Also listed were Model 703 Advantages:

Gas is exhausted behind the front sight rather than into the bolt group thereby reducing the frequency of cleaning.
Weapon can be slung over the shoulder in the assault-ready position.
Adjustable gas system compensates for ammunition and climatic conditions.
Rate of fire remains relatively constant (within ± 15 rounds/minute) at any setting.
The rifle’s upper receiver was very cumbersome when compared to that of the direct gas rifle although it weighed only 4 ounces more. All stats were comparable to the M16/M16A1. However, this rifle was never let out of the gate.

At the same time, there was another organization looking to change the direct gas operating system to a short stroke tappet system. The contract, DAAG25-68-C-0742, was awarded to Olin Mathieson Chemical Corporation’s Winchester Western Division with the mandate to create a system to retrofit existing upper receivers with “The object of this contract is to improve the combat and operating effectiveness and reliability of the existing M16A1 rifle by developing a short-stroke gas system.”

The program objectives consisted of:

Determining the functioning characteristics of 25 unmodified M16A1 rifles.
Designing a test rig to establish parameters for a short-stroke gas system.
Testing and developing two prototype short-stroke gas systems.
Conducting a 6,000 round test, with Government surveillance, on three Short-Stroke models for delivery to the Government.
Fabricating ten Short-Stroke Models for delivery to the Government.
Estimating contractor cost for retrofitting M16A1 rifles with short-stroke gas system.

The Colt Model 703 was built as a long stroke piston operation design. (Courtesy of Gary Paul Johnston)

“It is concluded from this program that the short-stroke gas system is functionally a practical system for the M16 Rifle and will significantly enhance the operating effectiveness and reliability of this rifle.”

The control rifles were 25 government furnished M16A1 rifles with the new and improved sliding weight buffers and the early non chrome lined bore/chambers. It should be noted a major issue with the reliability of the M16 in Vietnam was in fact due to the non-chroming of the chamber of the rifles. The corrosion and pitting caused a majority of the failures to extract, which were catastrophic in combat. The only way to remove the stuck cartridge case was to knock it out with a cleaning rod. Once the chamber was corroded there was not enough cleaning possible to correct the failure to extract. The barrel would have to be replaced.

The gas cylinder location was to be located 5.5 inches forward of the bolt face. The original gas port location was not used for several reasons including the need for a complete redesign and replacement of the already pinned on and costly front sight base. Additionally, a long flexible gas piston would be susceptible to damage in handling and usage, the gas piston falling outside of the handguard cap and forward portion of the handguard and probably most important was the gas to be tapped at an inconsistent low pressure portion of the pressure-time curve. The shorter gas system would permit a short and sturdy gas system and tap consistently high pressure portion of the pressure-time curve. The new gas block would be pinned to the barrel for stability and durability under the hand guard. The gas block consisted of gas cylinder housing, housing plug and utilized a plain piston. There was a new gas port drilled. The bolt carrier was modified with a solid carrier key that was reinforced by a carrier key pin that was copper brazed to the carrier key. The pin is inserted into the existing gas hole in the carrier and the carrier key screws were torqued and staked. A chamfered cam pin or a steel insert in the receiver is required to overcome peening of the receiver at the rear corner of the clearance cut for the cam pin rotation. This peening is the result of the cam pin impacting on the cam in the bolt carrier after bolt pickup and rotating into the receiver. The final design omitted the chamfer of the cam pin and opted for the steel insert, which is basically a steel screw that prevents the cam pin from damaging the aluminum receiver. There is a hole drilled in the upper receiver, on the left side where the hump is, where the cam slot is on the receiver and the screw inserted.

Shown is the way Winchester-Western dealt with the damage from the cam pin cutting into the receiver right behind the cam slot in the upper receiver by using a hardened steel screw. Colt would later adopt this concept with their steel insert they added to their receiver in late 2008 and would later be used on all of their monolithic upper receivers.

The result of the study concluded that material cost per rifle was $4.16 and labor costs (excluding burden) would be $4.13 per rifle. The value of machinery, tools and gages required (incl. benches, heat treatment and Parkerizing equipment) would be $2,064,000 and the contractor start-up costs would be $205,000. The conclusion of the contract is as follows:

The dynamic operating characteristics of the M16 rifle can be duplicated with a short-stroke gas system.
The short-stroke gas system will substantially eliminate those malfunctions associated with fouling, i.e. failure to fully open and fully close.
The M16A1 rifle with a short-stroke gas system is capable of firing a significantly larger number of rounds without cleaning and lubricating than the standard M16A1 rifle.
Information to date indicates that functionally the short-stroke gas system is a practical system for the M16A1 rifle.
Certain functional characteristics, including “fail to fire” and “fail to eject” are independent of the operating energy source (gas system), but are sensitive to other mechanical conditions.
A substantial amount of data has been generated that would be useful in further analysis of the M16A1 rifle system.

Like the results of the Colt Model 703 development, the retro fit program was quite viable. However, neither system was destined to replace that of the AR-15/M16 rifle. The findings of the numerous investigations showed what the root causes in the malfunctions were in Vietnam. In reality it was not a deficiency in the rifle system, in fact it was a good viable system destined to become one of the most prolific combat rifles of all time. It was found that the major problem, failure to extract, was caused by a combination of failing to chrome plate the chamber as well as lack of cleaning tools to maintain the rifle. Along with this there was no instruction on maintenance of the rifle. The other major issue was the untested and quantified decision to switch from IMR to Ball powder against the advice from both Gene Stoner and Colt’s. In a nutshell, the rifle could work reliably with ball propellant if it was in fact modified to do so, which is what was done: the change from a basic spring guide to a buffer that had sliding weights to prevent light strikes due to bolt carrier bounce and shot peening the bolts to increase its strength due to the higher cyclic rate that cause bolt breakage. Also, the firing pin was lightened and the disconnector strengthened. With all of these modifications there was little to no benefit in going backwards to the external pistol. The direct gas system would go on to prove itself in the late 1960s right up through the introduction of the M4. There was no further military interest in a new gas system. Although Rhino offered a retrofit kit for commercial sales in the 1980s, it never really gained momentum.

The lower handguard is removable on the LE1020 for the attachment of the M203 grenade launcher.

Right after the initial years of Gulf War 2, combat missions by Special Forces showed the newly introduced M4 carbine was about to go from a back-up rifle for those who had jobs that did not require the full length rifle but needed more than a pistol could provide, to a preferred primary combat rifle by SOCOM as well as regular Army. In the early days, SOCCOM began to experience failures of bolt breakage, and barrels rupturing, as well as failure to extract. Oddly enough, regular Army was not seeing the same issues. These reports let way into the HKM4/HK416 program that claimed to create a retrofit kit as well as a new rifle touted to “improve” combat reliability of the M4 by use of a short-stroke tappet operating mechanism. So literally the HKM4/HK416 upper receiver conversion was designed initially for SOCOM to help with their perceived problems with the M4. SOCOM had decided they needed their own rifle, not one that the Department of the Army was project manager of, which included the XM-29 program that would later be whittled down to the XM8, but one they fully controlled. SOCOM in no way wanted a weapon that was an Army weapons program – hence the commencement of the SCAR Special operations Combat Assault Rifle Program. It was also no secret that the Army did not want the SOCOM SCAR rifle; they wanted the XM8. With both rifle programs running consecutively, there was a race to get the SCAR rifle adopted before the Army could get the XM8 adopted. In the end, the XM8 program was cancelled. Any manufacturer of firearms who wanted a piece of the government sales knew the M4 was sole source from the U.S. Govt. to Colt and that the Army was happy. The only way to get into the U.S. military market was to say what they had was deficient and to open up the market to non-Colt M4 rifles.

Everybody knew that SOCOM was looking for an external piston operated rifle although it was not written that way. Colt was willing and ready to partake in the competition. They, however, would introduce and submit 3 rifles to the competition; two of which were their well proven and tested direct gas variations and the third would be Colt’s first external operated piston rifle since the Model 703 and that was in 1969. At the same time, both the government and Colt were looking into the issues with the rifle failures that SOCOM had. Was it a defect in the rifle design, quality, or was it user induced. Regardless of this outcome SOCOM wanted their own rifle.

Right side of the Colt Type-C SCAR rifle. Note the VLTOR modstock, the M1911-like ambidextrous selector lever, the Colt 1-piece upper receiver as well as the SOCOM mandated tan color. The gas system was held in place with a roll pin. One of the few criticisms SOCOM had on the rifle was the lack of ease of disassembling the operating system for maintenance and cleaning. This problem would be corrected on the next generation LE1020.

Colt submitted three rifles to the SCAR competition. Both types, A and B, were based on their combat proven direct gas system. The Type C was based on the short stroke tappet piston system. The system is simple and robust. There is an operating rod with a return spring attached, a piston and a gas cylinder sleeve. All this was held into the Type C rifle with a roll pin. Due to time constraints the roll pin was not removable for cleaning at the user level. The piston head had a tapered cone shape to it rather than the conventional flat shape. The upper receiver was the newly developed monolithic upper receiver with removable bottom rail to allow installation of the M203 grenade launcher. The carrier key was solid with no gas holes and part of the bolt carrier was machined to install the carrier key. The entire rifle was coated in UCT (Ultra Chem Tech), which is a nickel boron coating and then painted tan as per the specification. The piston operating system required an H2 buffer. The rifle used a unique and patented short throw ambidextrous selector lever and a Norgon ambidextrous magazine catch. There was an Otis Technology grip cleaning kit installed and a VLTOR modstock. The front sight base had a folding front sight that engaged and disengaged by a detent.

The Type C faired very well in the competition. Sources claim it was number two in the selection. The winner was the FN rifle. There were lessons learned by Colt from feedback on the Type C rifle that would assist them in further refinement of their piston rifle. The industry was calling upon Colt to produce an external piston operated rifle. There was growing demand in the international market for something to compete with the HK416.

The unique selector lever on the Type C SCAR as requested by SOCOM. U.S. Patent US 2005/0241470 A1 was granted on Nov. 3, 2005 to Colt’s Paul Hochstrate and the late Michael LaPlante. This was only used on the SCAR rifles.

In the secrecy that Colt is known for, they went to work on their next generation piston rifle in the 2007 time frame. This new rifle would be called the LE1020. The first major change was to have a way the operating rod and grooved piston could be removed at the user level for cleaning. On the front sight base was added a captive pin similar to that of the takedown and front pivot pin. This made it simple to remove and install. Also, to cut down on parts that could be lost, the piston was attached to the operating rod. The carrier key was simplified as well. A dowel pin was added so a standard bolt carrier could be used. The dowel pin went into a hole drilled into the gas slot in the top of the carrier and drilled into the bottom of the carrier key and the carrier key was held in place by two carrier key screws. The intent of the dowel pin was to take all of the stresses off of the carrier key screws. Barrels were offered in 10.5, 14.5 and 16.1 inches. The lower receiver went back to the standard lower receiver. The UCT finish was dropped in favor of standard Mil-Spec hard coat anodizing as well as manganese phosphate on the bolt carrier group and barrel. From 2007 to 2010, the LE1020 made its way into all the major trade shows in both the military and commercial arena. The LE1020 got a significant amount of attention from both attendees and press at SHOT Show and Colt was flooded with phone calls from customers regarding the high demand for the Colt piston rifle. At this time the controversy was in clear swing as to the question if the U.S. military is giving their warriors the best small arms available in the world. Colt’s resources were heavily involved in that fight. It is this author’s opinion that Colt higher management felt that if they introduced an external piston operated rifle, it would be an admission that their M4 was inferior. Perhaps in hindsight this author feels the better way to look at it would have been to give the customer what they asked for rather than give the business away to another company.

In 2009, Colt released for sale their first monolithic upper receiver but in the direct gas configuration called the LE6940. As expected, the rifle was an instant success. It also was the first new rifle introduced by Colt since the LE6920, which was
in the late 1990s.

In 2010, Colt introduced to the military market their refined LE1020 as the APC or Advanced Piston Carbine and for the LE market as the LE6940P. The biggest change between the LE1020 and the APC/LE6940P is the operating rod/piston. The new system was called an articulating link piston. This pivoting piston and op-rod connection compensates for deflection of the barrel during firing and due to thermal expansion. It also reduces recoil and muzzle climb during firing with the piston system closer to the bore. On the end of the piston are gas rings to seal the expansion chamber. The face of the piston is flat rather than tapered cone shaped as
was the LE1020.

The operating rod/piston, removed from the Type C SCAR upper receiver, is simple and robust. The operating rod would continue to be improved for the next two generations. The gas plug can be seen as well as the hole in the front sight base for the roll pin that held it in place. The rear sight was cut from a detachable carrying handle to make as compact and light as possible.

Added to the upper receiver is the steel insert behind the cam slot to prevent damage to the receiver. This is the same in concept to what Winchester-Western did in 1968 but Colt further refined the design to make it that much more effective
than the original.

After getting customer feedback, Colt modified the folding front sight base with a sliding lock. Customer comments came in that if they were to bump the front sight the sight would disengage causing them issues with engaging targets. Colt put a durable sliding lock that prevented this from happening.

Further testing on durability showed that the current bolt carrier design could be improved. Heavy usage showed that even with the dowel pin locating the bolt carrier key on the bolt carrier, upward recoil stresses on the carrier key screws allowed them and the dowel pin to break. Colt decided to switch to an integral bolt carrier/carrier key design with the impact area of the operating rod being on the face of the shortened integral carrier key portion of the bolt carrier. This eliminated this problem from happening.

The APC was submitted to the Individual Carbine competition. There were modifications to the lower receiver to make the rifle fully ambidextrous. The lower receiver was the new 7.62x51mm modular LE901 lower receiver with the ambidextrous bolt and magazine release. The front of the receiver was cut back to a 5.56mm configuration. The Colt entry was one of the finalists but the Individual Carbine program was cancelled due to nothing showing a significant increase in reliability over the standard M4. It is this author’s opinion that the testing in the IC program was flawed. They tested to the same 6,000 round protocol as they used for the M4. Of course they will see no major improvement. Perhaps if they pushed it out to 12,000 rounds they may have seen some significant improvements. But as the saying goes, the best is the enemy of good enough. According to the Army the M4 with its product improvement features is fully suitable for the Army. There is no requirement for anything to exceed those specifications.

The operating rod was modified to accept a larger return spring as well as the piston is now part of the operating rod. The gas cylinder is now removable by a detent pin identical to that of the takedown and pivot pins of the lower receiver.

The current APC/LE6940P rifles are what you have come to expect from Colt – durable workhorses that one can bet their life on. The APC will surely have a future in the international market and is in fact involved in testing in several countries. Unfortunately, it is the new kid on the block competing with piston operated AR rifles that have been in the field and refined for more than a decade. In time the Colt rifle will achieve a combat record and prove itself the way all of its predecessors have for the last 50 years, on the battle field.

Evolution of the piston rifle bolt carrier groups. On top is the Type C SCAR. This is UCT coated and notice the modifications required to install the carrier key. The second is also UCT coated but a simple sheer pin was added to the carrier key so any standard bolt carrier group could be used. The “skis” on the rear of the carrier were welded on the first two versions. The one shown is an early UCT coated carrier but production ones were made in standard manganese phosphate finish. The third carrier is a carrier designed specifically for the piston gun. The gas port holes removed and the “skis” machined as part of the carrier. The bottom is the final design that is a 1-piece carrier increasing the strength of the key thus extending service live dramatically.

Finalized gas system including the operating rod, the articulating link piston removed from the operating rod as well as the gas plug.

The LE1020 piston that is pyramid in shape. Bottom: The final production LE6940P uses a flat piston with the articulating link.

Current production LE6940P/APC (Advanced Piston Carbine). This refined LE1020 sports the proprietary Articulating Link Piston rod, the cam pin slot protector as well as the final production 1-piece bolt carrier. The rifle is offered in 10.5, 14.5 and 16.1 inch barrels as well as selective fire of semiautomatic only.

A historic depiction of the development of the Colt short stroke tappet system: starting on top was the original proof of concept designed by Art Daigle in a Greek hotel room. The second is the Type C SCAR system, the third is the Colt LE1020 system and on the bottom is the final LE6940P/APC system. Note the main changes had to do with the ease of disassembly and decreasing of number of components. The final two major designs was the change in the piston shape and then the implementation of the articulating link piston.

The captive pin that allows for the removal of the operating mechanism. This is probably the easiest of any of the piston operated rifles in the industry to remove/install. Start the pin inward with a cartridge or suitable tool and pull out to detent. The heavier return spring pushes the system right out of the front sight base where it is easy to grab and remove.

There has been a cultural shift from the 20-inch barrel length in the AR-15/M16 weapon systems chambered for the 5.56×45 NATO cartridge to progressively shorter barrels for the purpose of producing an increasingly more compact assault/entry weapon without resorting to a bull-pup design.  Simple usage of these short-barreled weapons has shown the necessity for both sound and flash suppression, the intensity of which (in exceptionally short barrel lengths) approached the intensity of a flash-bang diversion device.  This shift toward shorter barrels has resulted in the U.S. Army and Marine Corps adopting the 14.5-inch barreled M4 carbine with a re-design of the 5.56×45 from the 55 grain SS-109 to the 63 grain M855 ammunition to optimize this barrel length.  The differing bullet design also necessitated a change in the rifling twist rate from the original 1:12 inches to 1:7 inches.

Law enforcement and some special operation units have continued this trend by using weapons fitted with 10.5-inch barrels, and there is some misguided law enforcement interest (in these author’s opinions) in the M16 type weapons using 7-inch barrels.  Besides the horrendous flash and sound levels, these ultra short barreled weapons introduce significant ancillary issues, including weapon functioning and reliability as well as projectile stability and cartridge lethality.

One of the authors routinely measures entrance chamber pressures in his company’s suppressors to calculate safety factors for different standard barrel lengths. A special mount holding the Kistler 6215 piezoelectric sensor clamps over the suppressor where a 2.5 mm hole has been drilled through the suppressor wall. Shown is a 5.56mm suppressor on a 10.5-inch barreled M16 in a Lead-Sled rest.

In recent years, designers have become aware of limitations to suppressor structural integrity due to rapid pressure variations in the entrance chamber of their suppressors.  The entrance chamber is easily visualized as a simple cylinder that acts like a pressure vessel with a hole in the far end to control the rate of pressure decrease.  With gunfire, the pressure peaks almost instantaneously and drops literally in microseconds.  A lot of structural stresses are applied in this short time interval.  A firearm barrel can also be visualized as a pressure vessel, but one of varying length as the bullet progresses throughout its length.

Intuition has transitioned into the sound engineering practice of actually measuring pressures in the suppressor entrance chambers to calculate hoop stress and, by knowing the yield strength of the material, the safety factor.  While some of these factors can be approximated through calculations, actual measurements are definitive.  These issues are the subject of this paper.

Sound is generated by the sudden release of high pressure gases at the muzzle at the moment of bullet exit, and to adequately control (or reduce) the sound level, a suppressor must be designed to handle this pressure.  What has not been immediately apparent is the relationship between suppressor entrance chamber pressures and residual pressure in the bore of the firearm at the instant of bullet exit, and (by extension) the problems in suppressor design.  With finite element analysis for suppressor design becoming more prevalent, actual measured pressures will give far more accurate and believable information than pressures estimated (or calculated) from SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) peak chamber pressure tables.  SAAMI pressures are measured with specific chamber dimensions and ammunition, and not all chambers match the SAAMI chambers exactly, especially military chambers.

The piezoelectric sensor is shown in place on the 24-inch barrel with a safety U-clamp. The weapon is mounted in the machine rest ready for test firing.

The other question raised is one of kinetic energy of the bullet, which varies with the weight (mass) of the projectile and the square of the velocity.  Intuitively, one knows that the velocity will vary with the barrel length, but the actual variation cannot be easily determined from a single barrel length measurement.

The authors have serious concerns about these issues of reliability, function, lethality, and velocity as the barrel lengths decrease.  By examining bullet velocity, sound levels, and bore pressures as a function of barrel length, the authors hoped to correlate and determine optimal barrel length in response to current requests for short barreled M16 type weapon systems.

Concurrent to the desire to shorten barrels is the requirement for reducing weight of accessories, including sound suppressors.  While some suppressors are redesigned to utilize strong, lightweight alloys, others simply reduce weight by thinning structural elements, including the outer wall thickness.

Hoop stress is the name given to a calculation of forces that attempt to rupture a chamber or pressure vessel.  A silencer, especially the entrance chamber, is a pressure vessel that contains the pressurized gases for only an extremely short period of time.  The units of hoop stress are pressure units (psi) and are proportional to the diameter and highest peak internal pressure and inversely proportional to the wall thickness.

The piezoelectric sensor was screwed into its adapter, and the adapter was screwed into the last port in the barrel to obtain a peak measurement of bore pressure just as the bullet was exiting the barrel (uncorking).

The safety factor is the ratio of the yield strength of the material used for the silencer divided by the hoop stress.  At a safety factor of 1, 50% of the units will fail.  The military requirement is for a safety factor of 2 or more, and the aircraft industry requires a minimum safety factor of 2.5.  The safety factor is reduced when thinning of the wall of a suppressor, especially when combined with increasing the diameter.  The use of low yield strength materials, such as the prevalent 300 series corrosion resistant stainless steels, can result in an unsafe product, especially when considering the degradation in yield strength with the elevated temperatures of as little as 100-rounds rapid fire.

Further, since the instantaneous peak pressure in the entrance chamber of a suppressor is proportional to the pressure in the bore of the rifle at the instant of bullet exiting (uncorking) from the muzzle, it also follows that when used on short-barreled weapons chambered for the 5.56×45 NATO cartridge, pressures in the suppressor may well exceed the suppressor’s integrity, resulting in failure and possible injury.

Altering barrel length seriously affects not only weapon reliability, but also projectile velocity (including kinetic energy), non-suppressed sound levels, and flash intensity.  Short barreled gas operated weapons, of which the AR-15/M4 platform is of interest, pose increasing reliability errors with decreasing barrel length.  The prime reason is that as the barrel is shortened, there is decreased dwell time of the projectile in the bore after passing the gas port.  This makes timing more difficult, and adding muzzle accessories (such as a sound suppressor) will cause serious reliability issues, such as faster cycling.  Projectile gyroscopic stability depends on rotational velocity, which is determined by twist and linear velocity.  Instability will cause yaw (and keyholing) immediately on uncorking, which can damage suppressor parts and result in an unpredictable bullet path.

Five-round average pressure in the bore at the moment of bullet uncorking is plotted against barrel length. It is apparent that the pressure rises exponentially with decreasing barrel length. This rapidly increasing pressure produces significantly higher sound levels as well as flash and strain on suppressor design. The values for 10.5 and 14.5 inches were calculated by averaging the adjacent measurements.

The maximum velocity for the M855 projectile occurred in a 20-inch barrel. This is anticipated since the cartridge was designed specifically for this barrel length. Velocity drops rapidly as the barrel length decreases, especially below 10 inches where the velocity drops below 2,500 fps. M855 bullets traveling below 2,500 fps when impacting a target will not produce a lethal wound channel.

There have been a number of studies demonstrating the barrel external temperature gradient as a function of barrel length in addition to numerous studies of muzzle velocity as a function of length.  The authors are not familiar with studies relating length to measured (rather than calculated) bore pressures, especially port pressures at the instant of bullet exit (uncorking).

Experiment Setup
The experiment was to measure the bore pressure at the moment of bullet uncorking from the rifling for various barrel lengths.  It is common knowledge that actual direct reading piezoelectric port pressures are far more accurate than strain gauge measurements, especially since pressure measurements are desired only at the end of the barrel.  Once a piezoelectric sensor is calibrated, it can be used in many locations.  Conversely, a strain gauge must be recalibrated for each measurement location.

A 24-inch AR-15 barrel (1:7 twist) was obtained and prepared for the test.  The barrel used lacked a gas port for operating the mechanism, resulting in a single-shot weapon.  This was done to avoid inconsistencies caused by using some of the gas pressure to operate the action.  The barrel was marked by a partial cut in at 1-inch intervals starting at the far end and ending 5 inches from the bolt face.  For consistency, ammunition used was all military M855 ball, Lake City 2009, and all from the same lot.  Further, to assure consistency, the ammunition was stored in a cooler until it was loaded and fired.