Adoption thumb|From top to bottom: M16A1, M16A2, M4A1, M16A4. thumb|right|A Marine of the 3rd Battalion, 1st Marines cleans his XM16E1 in December 1967.
Adoption thumb|From top to bottom: M16A1, M16.A2, M4A1, M16A4 thumb|right|A Marine of the 3rd Battalion, 1st Marines cleans his XM16E1 in December 1967.