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.
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.
Here's an interesting article on barrel length and how it effects our short barrel AR ballistics.
http://sadefensejournal.com/wp/?p=1093
Conclusion
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 significant wound 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.
What does everyone think?
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