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.
The .45 is famous for being the bullet used in the M1911 and for being the choice of the U.S. Military for their pistols for almost 75 years. The .45 ACP has good stopping power because of the large size of the bullet, a .45 ACP bullet is almost half an inch. When shot at human targets, the .45 ACP bullet makes a big hole that causes blood loss and leads to faster death.