The .45 ACP is a type of bullet, made to be used in the M1911 pistol used by the U.S. Military. It was first made by John Browning in 1904, the "ACP" in the name stands for "Automatic Colt Pistol". It was the standard for handguns in the U.S. military until they switched to the Beretta M9 (Beretta 92) in 1985 that used a smaller caliber 9mm bullet.