The M16 uses the 5.56mm NATO (.223) caliber cartridge, with a muzzle velocity (the speed of bullet leaving the rifle) of over 900 meters per second (over 3,000 feet per second), and has a maximum effective range of 550 meters, with a rate of fire (how fast the gun shoots) of approximately 800 rounds per minute. The M16A1 can shoot semi automatic and fully automatic fire. The M16A2 can shoot semi automatic and three-round-burst fire. The M4A1 Carbine retains full automatic and semi automatic fire. The M16 normally holds 30 cartridges in its magazine, but there are variants of the magazine that hold only 20 or 10 cartridges.