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.
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.
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.
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.