The difference between .38 ACP and 9mm is that .38 ACP has a longer case (0.9 inches vs 0.754 inches) and is semi-rimmed, whereas the 9mm is rimless. Given the shorter case length, the 9mm Para also has higher chamber pressures. However, the Army wanted a .45, so the .45 ACP was devised and first used in the Model 1905 and eventually adopted in. Firearm Discussion and Resources from AR-15, AK-47, Handguns and more! Buy, Sell, and Trade your Firearms and Gear.