It is a shortened version of the M16A2 assault rifle.
The M4 carbine (officially Carbine, Caliber 5.56 mm, M4) is a 5.56×45mm NATO, gas-operated, magazine-fed carbine developed in the United States during the 1980s.
Gas-operated, closed rotating bolt, Stoner expanding gasĥ00 m (550 yd) (individual/point targets)