“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.
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 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.”
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