The M4 Carbine is a gas-operated, air-cooled, magazine-fed, selective fire, shoulder-fired weapon with a telescoping stock. A shortened variant of the M16 rifle, the M4 has a 14.5 in (368 mm) barrel, allowing the individual soldier to better operate in close quarters. The M4 has selective fire options including semi-automatic and three-round burst (like the M16), 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.