"n 1964, the M16 entered U.S. military service and the following year was deployed for jungle warfare operations during the Vietnam War.[1] In 1969, the M16A1 replaced the M14 rifle to become the U.S. military's standard service rifle.[17][18] The M16A1 improvements include a bolt-assist, chrome plated bore and a new 30-round magazine.[1]

In 1983, the U.S. Marine Corps adopted the M16A2 rifle and the U.S. Army adopted it in 1986. The M16A2 fires the improved 5.56×45mm NATO (M855/SS109) cartridge and has a new adjustable rear sight, case deflector, heavy barrel, improved handguard, pistol grip and buttstock, as well as a semi-auto and three-round burst only fire selector.[19][20] Adopted in 1998, the M16A4 is the fourth generation of the M16 series.[21] It is equipped with a removable carrying handle and Picatinny rail for mounting optics and other ancillary devices.[21]

The M16 has also been widely adopted by other militaries around the world. Total worldwide production of M16s has been approximately 8 million, making it the most-produced firearm of its 5.56 mm caliber.[22] The U.S. Army has largely replaced the M16 in combat units with the shorter and lighter M4 carbine,[23] and the U.S. Marine Corps approved a similar change in October 2015.[24]"