.38 ACP

Pistol cartridge designed by John Moses Browning

The .38 ACP (Automatic Colt Pistol), also known as the .38 Auto, .38 Automatic, or 9x23mmSR, is a semi rimmed pistol cartridge that was introduced at the turn of the 20th century for the John Browning designed Colt M1900. It was first used in Colt's Model 1897 prototype, which he did not produce. The metric designation for the round is 9×23mm SR (semi rimmed), which is not to be confused with other 9×23mm cartridges.

Initial loadings of this cartridge were quite powerful. Reported ballistics for the first commercial loads were a 130 grain bullet at , and some experimental loads ran as high as However, these ballistics proved too violent for the Colt Model 1900 pistol, and velocities were soon lowered to below Subsequent commercial loadings varied considerably in power. For example, Hugh B. C. Pollard, writing in Automatic Pistols in 1920, gives Winchester factory ballistics for a 130 grain bullet at muzzle velocity and of muzzle energy; for Ely ammo, the figures for a 128 grain bullet were and ; and for Kynoch a 130 grain bullet Later U. S. commercial loads in this caliber had factory standard ballistics of a 130 grain bullet at from the barrel of the Colt 1903 Pocket Model. With the United States Army Ordnance Corps favoring a return to a .45 caliber sidearm by the time the Colt autos in .38 ACP were introduced, the caliber never gained much popularity. However, they did see small but steady sales up until the introduction of the more powerful .38 Super, which was little more than the .38 ACP loaded back to its original ballistics. Sales of .38 ACP ammunition enjoyed a modest spike during the surplus gun boom of the 1950s, 1960s, and 1970s; since the cartridges would usually cycle in Spanish surplus pistols like the Astra 400 that were chambered for the 9×23mm Largo, even though the .38 ACP was semi rimmed and slightly shorter than the rimless 9mm Largo. Some Astra 400 pistols were stamped "9M/M&38" on the barrel, denoting that the barrel was specifically designed to chamber both 9mm Largo and .38 ACP. Europe would eventually favor the 9×19mm Parabellum cartridge. This cartridge is ballistically similar to the .38 ACP but utilizes a smaller case and higher pressures. Browning himself was not done with 9 mm cartridges and introduced the 9mm Browning Long in 1903 and the .380 ACP in 1908.

.38 Super was introduced in 1929, as a higher pressure loading of the .38 ACP. Even though .38 ACP and .38 Super are the same size, it is dangerous to use the more powerful .38 Super ammunition in a firearm intended for .38 ACP, as firearm damage may result. In the interest of safety, American ammunition companies formerly loaded .38 Super ammunition in nickeled cases exclusively. Since 1974, .38 Super cartridges have been marked with the +P markings used for greater pressure loads.