What does ACP stand for in the term "45 ACP"?

The .45 ACP is an abbreviated name for the .45 automatic colt pistol cartridge. John Browning designed the cartridge in 1904 for a semiautomatic pistol that he was working on, which later became the Colt model 1911.

The .45 ACP was designed after the U.S. Cavalry requested a new handgun that would use .45 caliber cartridges. It was introduced in 1911 alongside the model 1911 pistol, and it is still used by some special operations and U.S. tactical police groups today.

Different manufacturers, such as Federal, Remington and Winchester, produce different loads of .45 ACP, including hollow point and full metal jacket.