Originally Posted by
Renegade04
In 1984, Colt began work on a new carbine design called the XM4 combining the best features of the Colt Commando and later the M16A2 rifles.
In 1984, the first model was made, and it was tested in May 1985. The first models had an upper receiver with an A1 sight, and were given a shorter 11.5-inch barrel, but later ones were given a longer 14.5-inch barrel for the bayonet and the M203 Grenade Launcher. The second model was made in May 1986, and it was tested from May 1986 though May 1987; at the time it had an A2 Upper Sight, and it had the M16A2's 1:7 inch rifle twist, to use the heavier 62-grain M855 rounds. The extended barrel improved the XM4's ballistics, reduced muzzle blast and gave the XM4 the ability to mount a bayonet and the M203 grenade launcher. The XM4 was also given the cartridge deflector, as well as other minor refinements. In May 1991, the XM4 was renamed to the M4, and Colt made a manual.
The M4 was officially accepted into service by the U.S. military in 1994, and first saw action in the hands of U.S. troops deployed to Kosovo in 1999. Shortly after the M4 entered service, the carrying handle upper (A2) was ditched for the flat-top upper with M4 feedramps making it much more versatile for optics.