6.35mm

.25 ACP
.25 ACP cartridge with scale
Type	Pistol
Place of origin	Belgium
Production history
Designer	John Browning
Designed	1905
Specifications
Case type	Semi-rimmed, straight
Bullet diameter	.251 in (6.4 mm)
Neck diameter	.276 in (7.0 mm)
Base diameter	.278 in (7.1 mm)
Rim diameter	.302 in (7.7 mm)
Rim thickness	.043 in (1.1 mm)
Case length	.615 in (15.6 mm)
Overall length	.91 in (23 mm)
Rifling twist	1:16
Primer type	Boxer small pistol
Ballistic performance
Bullet mass/type Velocity Energy 35 gr (2 g) Safety [1] 1,100 ft/s (340 m/s) 94 ft·lbf (127 J) 35 gr (2 g) JHP 900 ft/s (270 m/s) 63 ft·lbf (85 J) 45 gr (3 g) JHP 815 ft/s (248 m/s) 66 ft·lbf (89 J) 50 gr (3 g) FMJ 760 ft/s (230 m/s) 65 ft·lbf (88 J)
Test barrel length: 2 in Source(s): Guns & Ammo Magazine

The .25 ACP (Automatic Colt Pistol) (6.35×16mmSR) is a semi-rimmed, straight-walled centerfire pistol cartridge introduced by John Browning in 1905 alongside the Fabrique Nationale M1905 pistol.

The cartridge was designed by John Browning for early blowback pistols that lacked a breech locking mechanism. The cartridge was designed to duplicate the performance of a .22 Long Rifle cartridge, when fired from a 2" barrel. The .25 caliber was the smallest case Browning could use that still had space for a primer pocket. The greased, coated lead bullet design, standard for the .22 long rifle of the day, was replaced with a copper jacketed round nose profile for more reliable feeding in self-loading pistols. The bullet weight was typically 50 grains, keeping with the sectional density of the 40 grain .22 caliber bullet. The cartridge is of semi-rimmed design meaning that the rim protrudes slightly beyond the diameter of the base of the cartridge so the cartridge can headspace on the rim. A recessed extractor groove allows an extractor to grab the cartridge reliably. It is one of the smallest centerfire pistol round in production, and is commonly chambered in small, so-called "vest pocket" pistols. The .25 ACP achieved widespread use after Colt introduced the Browning-designed Fabrique 1905 vest pocket (sometimes referred to as 1906) to the United States as the Colt Model 1908 Vest Pocket.

Though the .25 ACP was designed for semi-automatic pistols, various .25 ACP revolvers were produced in the early twentieth century by Belgian, French, and German gunmakers such as Adolph Frank and Decker. In the late twentieth century, Bowen Classic Arms produced a custom Smith & Wesson revolver in .25 ACP.

Following World War II, the Italian Lercker machine pistol was chambered for the .25 ACP, but achieved little distribution.