The M4A1 Carbine assault rifle is a variation on the M16A2 assault rifle (similar to the M16A1 model).
It is a gas-operated, magazine-fed, selective fire, shoulder-fired weapon with a telescoping stock and 14.5 in (370 mm) barrel to ease close quarters combat. Like the rest of the M16 family, it fires the .223 caliber, or 5.56 mm NATO round.
The M4 has selective fire options including semi-automatic and three-round burst (like the M16A2 and M16A4), while the M4A1 has the capability to fire fully automatic instead of three-round burst. The carbine is also capable of mounting an M203 grenade launcher (the M203A1 with a 9-inch barrel as opposed to the standard 12-inch barrel of the M203 used on the M16 series) as well as its successor, the M320 grenade launcher. The M4 carbine is heavily used by the U.S military.