<Tr> <Th> Feed system </Th> <Td> 20 - round detachable box magazine: 0.211 lb (96 g) empty / 0.738 lb (335 g) full 30 - round detachable box magazine: 0.257 lb (117 g) empty / 1.06 lb (480 g) full) Beta C - Mag 100 - round double - lobed drum: 2.20 lb (1,000 g) empty / 4.81 lb (2,180 g) full) </Td> </Tr> <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 5.56 mm automatic 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 30 - round magazine . </P>

What is the military version of the ar-15