M4A1

History

Following the adoption of the M16, carbine variants were also adopted for close quarters operations. The CAR-15 family of weapons served through the Vietnam War. However, these carbines had design issues. They were primarily made for close ranges, like submachine guns, and so weapons like the XM177 "Commando" series suffered at longer ranges. In 1988, Colt began work on an new carbine design called the XM4 based on their M16A2 rifle. To remedy the range problem, Colt incorporated a 14.5 inch barrel, giving the weapon greater reach than the XM177 while still being more compact than the M16. The barrel was given the improved 1:7 inch rifle twist of the M16A2 to use the heavier 62 grain M855 rounds. The extended barrel gave the XM4 the ability to mount a shortened M203 grenade launcher. In 1994, the U.S. military officially accepted the XM4 into service as the M4 Carbine to replace the M3 Grease Gun, as well as selected M9 pistols and M16A2 rifles.

The United States Marine Corps has ordered its officers (up to the rank of lieutenant colonel) and Staff Non-commissioned officers to carry the M4 carbine instead of the M9 handgun. This is in keeping with the Marine Corps doctrine, "Every Marine a rifleman". The Marines however chose the full-sized M16A4 over the M4 as its standard infantry rifle. United States Navy corpsmen E5 and below will also be issued M4s instead of the M9.

The M4 has largely replaced the submachine gun in military use due to increased use of body armor, as SMGs firing pistol-power cartridges are unable to penetrate modern body armor.