5.56mm NATO is a rimless, bottleneck, intermediate cartridge originally designed in Belgium by FN Herstal. It is nearly identical to .223 Remington. The 62 gr M855 is the standard ammunition used by the U.S. Military in the M4 carbine and M249 SAW. The round is fully jacketed and comes in a brass case. It has a muzzle velocity of 3,060 fps and produces muzzle energy of 1,289 ft/lbs.