The U.S. rifle, caliber 5.56 mm [223 Remington], M16, is a lightweight air cooled, gas-operated, magazine-fed, shoulder weapon. It is capable of both semiautomatic and full automatic fire. (background) The basic infantry rifle that replaced the M14 at the start of the Vietnam War. It was designed by Eugene Stoner for the Armalite Company and was produced by Colt. see Mattel.