These days the US miltary favors M4s with 14.5″ barrels and carbine-length gas systems, which is to say, a gas port located approximately 7.8″ in front of the bolt face. It works. On the civilian market, lots of companies are offering (usually pinned) 14.5″ barrels with the midlength gas system, i.e. gas port about 9.8″ ahead of the bolt face. Civilian shooters will tell you the midlength gas system is a softer shooting system. But is it more reliable? We could make arguments about it, but NSWC Crane decided to put it to the test.
Barrel Wear: Accuracy Degradation
In a normal, carbine-length gas system, a degradation of accuracy can be seen after about 6,000 rounds. Crane’s testing found that after 12,000 rounds, the midlength barrels did not suffer appreciable accuracy degradation.
Does the midlength gas system cause any loss in velocity? In the unsuppressed case, the carbine had a mean muzzle velocity of 2,905.4 feet per second, and the midlength had a muzzle velocity of 2,906.4 feet per second. The difference in means is -1.0 feet per second, or 0.04%.
In the suppressed case, average muzzle velocity was 2,989.7 feet per second for the carbine system and 2,983.0 feet per second for the midelength. Here the difference is 6.7 feet per second, or 0.23%. In either case, muzzle velocity is negligible.
Terminal Velocity (100 yards)
Given the negligible difference in velocity at the muzzle between the carbine length and midlength gas systems, we would expect the difference at 100 yards to be similarly negligible. And it is. In the unsuppressed case, carbine-length gas system yields a terminal velocity of 2,635.9 feet per second and the midlength gives 2,677.6 feet per second for a difference of 41.6 feet per second or 1.57%.
The suppressed case is similar, with the carbine-length gas system providing 2,654.1 fps and the midlength providing 2,686.7 fps, for a difference of 32.6 fps or 1.22%. Overall, there’s not much of a performance difference. Midlength is actually slightly better. Of course, we wouldn’t expect much of a difference in velocities from changing the gas system length.
We might expect a change in cyclic rate from altering the gas system length (and hence the gas pressure in the system). Also, small reductions in cyclic rate tend to be good from a parts-life standpoint as well as a controllability standpoint. So what do we get? Again, we have the suppressed and unsuppressed cases. In the unsuppressed case, we see a reduction of 127.2 rounds per minute or 15.9% going from a carbine-length 864.8 rpm to a midlength 737.6 rpm. The suppressed case gives us a reduction of 62.7 rounds per minute or 6.9% when we go from a carbine-length 944.2 rpm to a midlength 881.5 rpm.
Given the reduced cyclic rate for the midlength, does this translate into more reliability? The tests showed that it did. The carbine length gas system had 65 stoppages, and the midlength gas system had 30. The high temperature (160 F) testing phase accounted for 5/65 stoppages in the carbine-length gas systems and 1/30 stoppages in the midlength gas systems. Low temperature testing (-60 F) accounted for 27/65 stoppages in the carbine-length gas systems and 15/30 stoppages in the midlength gas systems. All other stoppages occurred in ambient-temperature testing.
So now we have some hard data proving that midlength gas systems are better on 14.5″ barreled weapons. Great! This gets more interesting because the USAF is looking at buying 50,000 improved M4s for security forces, battlefield airmen and OSI. That’s the kind of significant buy that can have an impact on what kind of weapons the rest of the US armed forces use. Remember, the Air Force were the first to embrace the M16.