The .38 ACP (Automatic Colt Pistol) also known as the .38 Auto was introduced at the turn of the 20th century for the Browning designed Colt M1900. The cartridge headspaces on the rim. It had first been used in his Model 1897 prototype, which Colt did not produce. The metric designation for the round is 9×23mmSR (SR—Semi Rimmed) (not to be confused with the modern 9×23mm Winchester or the .380 ACP).