The M4 carbine (officially Carbine, Caliber 5.56 mm, M4) is an assault rifle developed in the United States during the 1980s. It is a shortened version of the M16A2 assault rifle. The M4 is extensively used by the US military, which decided to largely replace the M16 rifle in US Army (starting 2010) and US Marine Corps (starting 2016) combat units as the primary infantry weapon[7][8] and service rifle. The M4 has been adopted by over 60 countries worldwide,[9] and has been described as "one of the defining firearms of the 21st century".[10]

Since its adoption in 1994,[9] the M4 has undergone over 90 modifications to improve the weapon's adaptability, ergonomics and modularity, including: the M4A1, which possesses a thicker barrel and a replacement of the burst-fire control group with a fully automatic one; the SOPMOD, an accessory kit containing optical attachments; and the underbarrel weapons such as M203 and M320 grenade launchers to the Masterkey and M26-MASS shotguns.

In April 2022, the U.S. Army selected the M7 rifle, a variant of the SIG MCX Spear, as the winner of the Next Generation Squad Weapon Program to replace the M16/M4.[11]

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