The .25 ACP (Automatic Colt Pistol), also known as the .25 Auto, .25 Automatic, or 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 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.^[2]

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.^[3] In the late twentieth century, Bowen Classic Arms produced a custom Smith & Wesson revolver in .25 ACP.^[4]

The use of the .25 ACP allows for a very compact lightweight gun, usually pocket pistol. With quality ammo available on the market, the .25 ACP is viewed by some^[who?] as a solid choice for personal defense handguns due to its centerfire-case design, which is inherently more reliable than a .22 LR rimfire cartridge.^[5]

• FN 1905/6
• Modern jacketed hollow point loads for 6.35mm/.25 cal.
• The Walther Model 9 pistol uses 6.35mm
• Bauer .25 Auto pocket pistol
• Raven MP-25 .25 ACP chrome with faux mother of pearl grips and push up safety
• Colt Model 1908 Vest Pocket used .25 ACP

• 6 mm caliber – Firearm cartridge classification
• Table of handgun and rifle cartridges