.380 Auto, also known as 9x17mm, is a semi-rimmed cartridge designed by John Browning in 1908 for use in semi-automatic pistols. It has a bullet diameter of 0.355 inches (9mm), a case length of 0.680 inches (17mm), and a rim diameter of 0.374 inches (9.5mm).
ACP stands for Automatic Colt Pistol and refers specifically to the .45 ACP cartridge, also designed by John Browning. The .45 ACP cartridge has a bullet diameter of 0.451 inches (11.43mm), a case length of 0.898 inches (22.81mm), and a rim diameter of 0.480 inches (12.2mm).
Therefore, .380 Auto and .45 ACP are distinct cartridges with different dimensions and performance characteristics.