Barrel Length Studies in 5.56mm NATO Weapons

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.

Out of curiosity (on a separate barrel) bore pressure measurements were made at a port drilled three inches from the bolt face. A bore pressure at this point was in excess of 55,000 psi.

A 2.5mm port was drilled 1/2 inch from the muzzle and threaded to accept a Kistler 6215 direct-measuring piezoelectric pressure sensor in a short adapter.  Five shot strings were measured using a Kistler 5015 charge amplifier and the results averaged.  Since the meter records the highest peak pressure it is exposed to, the only peak it will see is when the bullet is less than 1/2 inch from the muzzle, and this will represent the bore pressure at the moment of bullet uncorking.  On each of the five shots in the string, velocity measurements were made five feet from the muzzle and absolute sound pressure levels were recorded using a Larson-Davis 800B sound pressure level meter at the reference location and protocol specified in Mil-Std 1474D (1 meter to the left of the muzzle, 90 degrees to the bore axis).  The weapon was held in a machine rest for consistency.

The barrel was then shortened an inch at a time with drilling a new pressure port 1/2 inch from the new muzzle and the piezoelectric sensor was moved to the new location.  The previous measurements were repeated and so on.  The last series of measurements were made when the barrel was 5 inches long.

To satisfy curiosity, on another barrel, a port was placed at 3 inches from the bolt face and port pressures were measured and averaged over a 15-round sample.

Data and Analysis
Measurements for barrel lengths of 24 down to 5 inches showed that the pressure in the barrel at the moment of uncorking varied from 4,800 psi for the 24-inch barrel to over 25,000 psi for the 5-inch barrel.  This is represented in Graph 1 and summarized in Table 1.  Plotting the logarithm of the uncorking pressure against the barrel length produces a relatively straight plot, indicating that the pressure rises exponentially with shortening of the barrel.

All data (velocity, peak bore pressure at the port, and sound pressure level) was recorded for each shot, and the average of the five shots was calculated for each measurement parameter. Shown is Dr. Philip Dater, one of the co-authors, as the scribe.

The pressure we measured at the port three inches from the bolt face was 55,744 psi usingM855 (average of 9 rounds).  There was some round-to-round variation with variations from a low of 52,500 psi to a high of 57,600 psi.  Interestingly, the round-to-round variation in pressure was less pronounced at the more distal ports.  We do not have the SAAMI pressure measurements for chamber pressure in the M855 5.56x45mm ammunition, but for commercial .223 Remington, the average maximum SAAMI pressure is listed at 55,000 psi.

Because these weapons are used more and more frequently with sound suppressors, it is interesting to note the uncorking pressure at the more common barrel lengths of 14.5 inches and 10.5 inches.  Pressure data for the 14.5 and 10.5 inch barrels was approximated by averaging the pressure between the two adjacent measurements (14 and 15 inches, etc.) yielding pressures of 8,150 and 11,500 psi respectively.  There is a passing interest in the non-serious user for suppressing the M16 with a 7-inch barrel, and the uncorking pressure at that point is 17,140 psi, approximately 50% higher than it was for the 10.5-inch barrel, which is itself approximately 50% higher than the 14.5-inch barrel.

One of the authors has measured port pressures in the entrance chamber of one of his company’s 5.56mm suppressors with both a 14.5 inch and 10.5 inch barreled HK416, and there was a 50% increase in suppressor chamber pressure of the 10.5 inch barreled weapon as compared to the longer 14.5 inch version. This correlates well with the difference in bore pressure at the instant of bullet uncorking.

Barrel preparation consisted of scoring the barrel at 1-inch intervals with a parting tool and drilling/threading the port for the sensor. All but the last port (where the pressure sensor was installed) were blocked with a gas-tight setscrew that was modified to completely block the port.

The M855 ammunition is optimized for a 20-inch barrel with a 1:7 twist barrel.  It was not surprising that the greatest velocity of 2,979 ft/sec was obtained in the 20-inch barrel, and the lower velocity on barrels greater than 20 inches is explained by decreased pressure driving the bullet no longer exceeding the slowing from friction.  After all, Eugene Stoner designed the cartridge for the 20 inch barrel.

The sound pressure level was measured according to Mil-Std 1474D, which specifies A-weighting.  Weighting degrades meter performance to match the frequency response of the human ear, and A-weighting is accurate and appropriate only for sound levels below 55 dB.  For sound levels above 130 dB, and in particular in the 160+ dB region of the non-suppressed 5.56mm rifle, the measurements should be performed without any weighting (also called “linear” or Z-weighting, depending on the meter manufacturer’s designation).  While there is a rough correlation using A-weighting between uncorking pressure and measured sound level, the sound measurements are not considered overly accurate due to compliance with the Mil-Std.

Sound levels are pressure measurements expressed as a logarithmic ratio of the actual pressure referenced to 20 micro-Pascals, the threshold of human hearing.  There was a little less consistency in the sound measurements than in actual uncorking barrel pressure measurements, partially because of adding several more variables.  These included the acoustic impedance of the air and wind direction/velocity.  In addition, the inaccuracies in this sound intensity range by using the called-for A-weighting introduces some level of inaccuracy that would probably not be seen in unweighted measurements.  When pressure is plotted against sound pressure level in decibels and sound pressure level is plotted against barrel length, there is slightly more deviation from the projected average, but the trend and general correlation is statistically meaningful.  Actual sound pressure levels varied from 162.5 dB(A) in the 24-inch barrel to 165.1 dB(A) in the 5-inch barrel.

Equally illuminating in this study was the correlation between velocity and barrel length (see Graph 2).  To generate a lethal wound channel, the M855 projectile must have a velocity of at least 2,500 ft/sec on impact with the target.  Below that critical velocity, the M855 bullet simply drills a 1/4 inch hole in the target, which too frequently is not lethal unless it passes through a vital structure.  Some of this limitation is being addressed with newer projectiles not available to the authors at the time of the study.  In the longer barrels, the maximum velocity of 2,979 ft/sec was in the 20-inch barrel with a velocity of approximately 2,700 ft/sec in the 14.5-inch barrel.  The critical velocity of 2,500 ft/sec was in a barrel between 9 and 10 inches in length, which further shows the folly of considering a 7-inch barrel for this cartridge.

The weapon with the pressure sensor in place is shown in the machine rest. Forward of the muzzle are the screens for velocity measurement, and to the left of the weapon is the microphone for sound pressure measurement. The microphone is located one meter to the left of the muzzle in the reference location referred to in Mil-Std 1474.

To satisfy the curiosity of the authors about the effects of barrel length in the 5.56×45 NATO weapons, an experiment was crafted to measure actual bore pressure in the barrel at the moment of projectile exit, velocity, and sound pressure level with a barrel length varying between 24 and 5 inches.  This has practicality on multiple levels.

When considering sound suppression of this cartridge, a suppressor has to be designed to handle the pressure of the gases presented at the instant of bullet exit, and higher uncorking pressures necessitate a larger suppressor to handle the gas load presented.  In separate studies, the authors have noted that the pressure gradient is not uniform throughout the entrance chamber of a suppressor due to the forward motion of the gases.  This indicates that a larger volume entrance chamber needs to rely on increased length rather than diameter.  With higher uncorking pressures, there is also increased erosion of the suppressor’s blast baffle from the superheated, partially burned powder particles functioning like a plasma torch.  Further, increasing diameter necessitates heavier walls to keep from increasing hoop stress (and decreasing safety factor) with the additional result of a physically heavier suppressor.  To attempt to preserve sound reduction performance, a suppressor will need to be longer (and heavier) with a shorter barrel, negating most of the compactness gained by barrel shortening.

Secondly, with shorter barrels, tuning of the gas port for weapon cycling becomes far more critical.  Adding a suppressor, which does slightly increase bore pressure, will result in more erratic and forceful cycling of the weapon leading to earlier weapon failure.  It is necessary to remember that the 5.56×45 NATO cartridge was designed specifically for a 20-inch barrel on a gas operated weapon with 7 inches of dwell time after the gas port.  The 14.5-inch M4 barrel retains the 7 inch dwell length after the gas port.

Lastly, decreased velocity with barrels much shorter than 14.5 inches have a number of unwanted effects.  Lowered linear velocity produces lower rotational velocity, which will result in diminished gyroscopic stability of the bullet.  It will also result in significantly decreased projectile kinetic energy, decreased ability to generate a sig nificant would channel, and will reach a point of diminishing returns where lethality of the projectile definitely comes into question.

Thus, it is the opinion of the authors that barrel lengths less than 14.5” in this caliber introduce effectiveness issues that may be detrimental to the user.