What does acp mean?

ACP stands for Automatic Colt Pistol. It is used to differentiate two bullets of the same caliber when one is for a pistol and one is for a revolver. For instance, .45 Colt is for a revolver, and .45 ACP is for a pistol. Other than the .45 inch measurement, the two have nothing in common and are not interchangeable; the .45 ACP cannot be used in a revolver chambered for .45 Colt and vice versa. The cartridge length and capacity are just two of the factors that make each different from the other.