<Tr> <Th> Sights </Th> <Td> Iron sights or various optics </Td> </Tr> <P> The M16 rifle, officially designated Rifle, Caliber 5.56 mm, M16, is a United States military adaptation of the Armalite AR - 15 rifle . The original M16 was a select - fire, 5.56 × 45mm rifle with a 20 - round magazine . </P> <P> In 1964, the M16 entered U.S. military service and the following year was deployed for jungle warfare operations during the Vietnam War . In 1969, the M16A1 replaced the M14 rifle to become the U.S. military's standard service rifle . The M16A1 improvements include a bolt - assist, chrome plated bore and a new 30 - round magazine . </P> <P> 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 . Adopted in 1998, the M16A4 is the fourth generation of the M16 series . It is equipped with a removable carrying handle and Picatinny rail for mounting optics and other ancillary devices . </P>

When did the us military started using the m16