Automatic Colt Pistol

Automatic Colt Pistol (ACP) is a series of firearms cartridges designed by John Moses Browning, for use in the semi-automatic pistols he designed for Colt and Fabrique Nationale. The five cartridges were introduced between 1899 and 1908.

All these cartridges are straight-sided and appear visually similar, being mostly scaled versions of each other. Other than the size, the main difference is that .25 ACP, .32 ACP, and .38 ACP are semi-rimmed and headspace on the rim, whereas .380 ACP and .45 ACP are rimless and headspace on the mouth of the case.