The .38 AMU Ammo is a special cartridge modified by machining the rim and inserting it in an extension groove to make it semi-rimless to achieve better functionality. The original intention was to improve the .38 AMU Ammo's accuracy in the .38 Caliber Ammo so that it could compete in the NRA Precision Pistol matches and meet their specific requirements. Since the competition format is a sustained fire stage, the capacity of the pistol or revolver needs to contain at least five rounds. The larger bore of the .38 AMU Ammo is desirable over the .32 Caliber Ammo since the bigger the size, the higher the chance of nailing more points to win the competition. This bullet was designed to solve the jamming problems in the modified Colt M1911 pistols through its semi-rimless version of .38 Caliber Ammo.
Herman Gano designed the .38 AMU Ammo, and the prototype was developed by Federal, Winchester-Western, and Remington.
Due to its big-bore creating a bigger hole in the targets and high accuracy, the .38 AMU Ammo was very popular in competitive target shooting.