|Place of origin||United States|
|Case type||Semi-rimmed, straight|
|Bullet diameter||.356 in (9.0 mm)|
|Neck diameter||.384 in (9.8 mm)|
|Base diameter||.384 in (9.8 mm)|
|Rim diameter||.406 in (10.3 mm)|
|Rim thickness||.050 in (1.3 mm)|
|Case length||.900 in (22.9 mm)|
|Overall length||1.280 in (32.5 mm)|
|Primer type||Small pistol|
|Source(s): Cartridges of the World |
The .38 ACP ( Automatic Colt Pistol) also known as the .38 Auto 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 9×23 mm SR (Semi Rimmed) (not to be confused with the later 9×23mm Winchester).
Initial loadings of this cartridge were quite powerful. Reported ballistics for the first commercial loads were a 130-grain bullet at 1,260 ft/s (380 m/s), and some experimental loads ran as high as 1,350 ft/s (410 m/s).  However, these ballistics proved too violent for the Colt Model 1900 pistol, and velocities were soon lowered to below 1,200 ft/s (370 m/s). 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 1,175 ft/s (358 m/s) muzzle velocity and 398-foot-pound-force (540 J) of muzzle energy; for Ely ammo, the figures for a 128-grain bullet were 1,100 ft/s (340 m/s) and 344-foot-pound-force (466 J) and for Kynoch a 130-grain bullet 1,000 ft/s (300 m/s). Later U.S. commercial loads in this caliber had factory standard ballistics of a 130-grain bullet at 1,040 ft/s (320 m/s) from the 4.5-inch (110 mm) barrel of the Colt 1903 Pocket Model.  With Army Ordnance 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 and 1960s, since the cartridges would usually cycle in Spanish surplus pistols like the Astra 400 that were chambered for the 9×23mm Largo, despite the fact that the .38 ACP was semi-rimmed and slightly shorter than the rimless 9 mm 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 mm Luger cartridge. The Luger was ballistically similar to the .38 ACP but utilized a smaller case and higher pressures.
.38 Super was introduced in the late 1920s 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.
- Colt M1900
- Colt M1902
- Colt M1903 Pocket Hammer
- Star Model AS
- Webley-Fosbery Automatic Revolver
- Webley Automatic Pistol
- M1911 pistol (civilian market)
- Barnes, Frank C. (2006) . Skinner, Stan, ed. Cartridges of the World (11th ed.). Iola, WI, USA: Gun Digest Books. pp. 328, 338. ISBN 0-89689-297-2.
- Article in the April 19, 1900, issue of Shooting and Fishing, quoted in Belden, C.T and Haven, A History of the Colt Revolver (1940)
- Sapp, Rick (2007). Standard Catalog of Colt Firearms. Iola, Wisconsin: F+W Media, Inc. pp. 130–131. ISBN 0-89689-534-3.