Question: what does acp mean in 45 caliber?
Answer: 45 ACP (Automatic Colt Pistol) or . 45 Auto (11.43×23mm) is a handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it was adopted as the standard chambering for Colt's M1911 pistol.