M4 / M15

The M4 carbine was developed in the United States between 1982 and 1993 as a shortened version of the M16A2 assault rifle. In 1982, the U.S. Government requested Colt to develop a carbine variant of the M16A2, then designated the Colt 645. The resulting design, initially titled the XM4, underwent testing by the Army’s Armament Research and Development Center in 1983. Following revisions to the Quick Reaction Program in 1984, the Army formally approved the carbine's development. Production prototypes were contracted to Picatinny Arsenal in 1985, and the design was finalized in 1987. Colt, FN Herstal, and Remington Arms have served as manufacturers for the platform.

The M4 is a gas-operated, air-cooled firearm utilizing a rotating bolt and a bolt carrier that reciprocates into a receiver extension aligned with the barrel. Its operating system employs an internal piston, based on the Stoner bolt and carrier design. The receiver is constructed from forged 7075-T6 aluminum, while the barrel, bolt, and fire control group consist of steel. The weapon features a flattop upper receiver equipped with a Picatinny rail for mounting optics and a removable carrying handle. Standard furniture, including the handguard, pistol grip, and multi-position telescoping buttstock, is manufactured from reinforced plastic. The design maintains 80% parts commonality with the M16A2 rifle but utilizes a 14.5-inch barrel and a shortened buffer assembly.

The M4 entered U.S. military service in 1994 and saw its first operational deployment in Kosovo in 1999. It subsequently served as the primary infantry weapon during operations in Iraq and Afghanistan. The U.S. Army began replacing the M16 with the M4 in combat units in 2010, followed by the Marine Corps in 2016. The platform has been adopted by over 60 countries. Primary variants include the M4A1, which replaces the three-round burst with a fully automatic trigger group and often features a heavier barrel profile for sustained fire. Specialist configurations include the Mk 18 Close Quarter Battle Receiver with a 10.3-inch barrel and the Mk 12 Special Purpose Rifle. The Special Operations Peculiar Modification (SOPMOD) program provides standardized accessory kits for the platform. In 2022, the U.S. Army selected the M7 rifle to begin replacing the M4 under the Next Generation Squad Weapon program.