M4A1

Info
The M4A1 carbine is a fully automatic variant of the basic M4 carbine intended for special operations use. The M4A1 has a "S-1-F" (safe/semi-automatic/fully automatic) trigger group, while the M4 has a "S-1-3" (safe/semi-automatic/3-round burst) trigger group. The M4A1 is used by almost all U.S special operation units including, but not limited to, the Army Rangers, Army Special Forces, Navy SEALs and Air Force Combat Control Teams. The M4A1 is especially favored by counter-terrorist and special forces units for close quarters combat because of the carbine's compactness and firepower. These features are also very useful in urban warfare. It has a maximum effective range of about 500 to 600 meters (550–660 yd).

All U.S. Army forces will begin replacing their basic M4 carbines and all 600,000 M16 rifles with the M4A1 variant in 2014. The M4A1 in turn would likely be replaced with the Individual Carbine. Replacement of the M16 would come from the Individual Carbine instead of the M4A1 if procurement was attained.

In the last few years, M4A1 carbines have been refit or received straight from factory with barrels with a thicker profile under the handguard. This is for a variety of reasons such as heat dissipation, which is useful due to the complaints of high-heat production from test soldiers, which occurs during full-auto and accuracy as a byproduct of barrel weight. These heavier barrel weapons are also fitted with a heavier buffer known as the H2. Out of three sliding weights inside the buffer, the H2 possesses two tungsten weights and one steel weight, versus the standard H buffer, which uses one tungsten weight and two steel weights. These weapons, known by Colt as the Model 921HB (for Heavy Barrel), have also been designated M4A1, and as far as the government is concerned the M4A1 represents both the 921 and 921HB.

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GTA 4