Barrel Length Studies for the 5.56 NATO

From the article: "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."

The Firearms Blog has linked to a 2012 study published at the Small Arms Defense Journal analyzing the impact of various barrel lengths on muzzle velocity, pressure, bullet stability, and so on. The authors write:

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.

  (Underline added). They also report:

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.

The authors determined, as expected since this was the original design, that the 20-inch barrel gave the best performance, and anything less than 14.5 inches was asking for trouble. "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." However, at shorter lengths, the velocity falls below that necessary to inflict lethal wounds, and the pressures at the muzzle create a lot of stress on flash or sound suppressors, besides the issues of timing.