 This 5.56mm NATO round was even lighter and smaller than the Soviet 7.62×39mm AK-47 cartridge, but possessed higher velocity.In U.S. service, the M16 assault rifle replaced the M14 as the standard infantry weapon, although the M14 continued to be used by designated marksmen.Although at 20", the barrel of the M16 was shorter than that of the M14, it was still designated a "rifle" rather than a "carbine", and it was still longer than the AK, which used a 16" barrel.