M4A1 Carbine with M203 Combatbrick

"The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16 rifle, all based on the original AR-15 rifle designed by Eugene Stoner The M4 is a shorter and lighter variant of the M16A2 assault rifle. It is a gas-operated, magazine-fed, selective fire, shoulder-fired weapon with a telescoping stock. A shortened variant of the M16A2 rifle, the M4 has a 14.5 in (370 mm) barrel, allowing its user to better operate in close quarters combat. 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 (like the M16A1 and M16A3). 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). The M4 carbine is heavily used by the U.S military." -Wiki