The M4 carbine is a shorter and lighter variant of the M16A2 assault rifle. The M4 is a 5.56x45mm NATO, air-cooled, direct impingement gas-operated, magazine-fed carbine. It has a 14.5 in (370 mm) barrel and a telescoping stock. The M4 carbine is heavily used by the United States Armed Forces and is replacing the M16 rifle in most United States Army combat units.The M4 is also capable of mounting the M203 grenade launcher.