The very first handgun I ever had was a Sig Sauer P250. The trigger pull is a mile long and it is chambered in .45 ACP.
ACP means Automatic Colt Pistol.
If you only came to find out what it stood for, there you go. If you want to learn a little bit more then read on!
John Moses Browning, of Browning Arms Company fame noticed while hunting that when his friend shot in tall grass, the blast would push the grass with the force of expelling the bullet. From that experience he began to work on an idea to harness that force into helping to operate the firearm mechanisms to cycle the next round.
The creator of the 1911 pistol, the Browning Automatic Rifle (BAR), and the Browning Machine Gun (.50 BMG), sold his 32 Browning self loading cartridge to Colt which then became the .32 ACP (Automatic Colt Pistol)