What Does “ACP” Stand For?

One of the questions I was asked recently when mentioning the .45 ACP cartridge, is “what does ‘ACP’ stand for?” It stands for “Automatic Colt Pistol”. 

These include the following cartridges:

John Moses Browning
  • .25 ACP
  • .32 ACP
  • .38 ACP
  • .380 ACP
  • .45 ACP

If we stop right here, you now know what the acronym “ACP” stands for, but you miss out on a very important point as to why those cartridges get that designation. 

Browning Gas-Operated Machine Gun

We have the world’s most famous gun-maker, John Moses Browning, to thank for this designation. One day back in 1889, Browning was out shooting with some friends. Back in 1889, the only handgun types available were revolvers, the ubiquitous “six-shooter”. He made an observation that forever changed the course of firearms history. His friends took turns shooting. But when one of his short (vertically challenged) friends began shooting, Browning noticed the tall grasses just beyond the gun’s muzzle began to wave back and forth every time his friend fired his gun. Browning recognized this to be the energy exiting the muzzle from the exploding gasses produced when the gun was fired. This gave Browning an idea to harness that energy and not let it go to waste into the atmosphere. His idea was to use that energy to cycle the next round into the chamber. By the end of the day, he had a working prototype for his own self-loading pistol.

45 Colt (l) vs 45 ACP (r)

In 1890, Browning filed for a patent on a gas-operated machine gun. But Browning was an inventor, not a manufacturer. So, in 1895, Browning entered into a long-standing collaboration with the Colt Manufacturing Company, which began production of Browning’s gas-operated machine gun. And because Colt made numerous types of firearms, both self-loading pistols and manual-loading revolvers, they needed to distinguish the ammunition designed for the self-loading pistols from ammunition for manually loaded revolvers. Collectively, Colt gave the name “Automatic Colt Pistol” to those ammo calibers designed for self-loading, or automatic guns.