The M4 a lightweight, gas operated, air-cooled, magazine fed, select-fire rifle that is used by all branches of the United States military as a standard offensive and defensive small-arm, as well as several other of the world's armed forces in a number of variants. The rifle is chambered for the 5.56x45mm NATO cartridge (which means that it can fire the relatively common .223 Remington round as well) and boasts a cyclic rate of about 800 rounds per minute. Derived from the M16 rifle, the M4 is significantly lighter and shorter than its predecessor and is usually equipped with a collapsible stock and a 14.5 inch barrel. The rifle has a maximum effective range of 600 meters, though it was primarily designed for engagements at 300 meters and under. Like the M16, the M4's qualities lie in its user-friendliness, ease of functionality, and lack of complexity. Generally, a user will only need to access about three to four of the weapon's components during a routine maintenance. The weapon is also praised for its accuracy and its low recoil. This benefits the user in several ways: aimed fire will have a higher likelihood of striking a target and the reduced felt recoil affords the shooter the luxury of accurate and more rapid follow-up shots when engaging multiple hostiles.
- M4: The original M4 is capable of semi-automatic and three-round burst mode.
- M4A1: The newer M4A1 is capable of semi-automatic and full-automatic fire.
- Mark 18 CQBR: The Mark 18 CQBR (Close Quarters Battle Receiver) is a modified M4A1 with a 10.3 inch barrel upper receiver.