Test barrel length: 3.75 Source: Federal Cartridge 
The .380 ACP (Automatic Colt Pistol) (also referred to as the "9 mm Short", "9 mm Browning", "9 mm Kurz", "9 mm Corto", "9x17mm") pistolcartridge is a rimless, straight-walled pistol cartridge developed by firearms designer John Browning. It was introduced in 1908 by Colt, and has been a popular self-defense cartridge ever since.
The .380 ACP cartridge was designed for early blowback pistols which lacked a barrel locking mechanism. The locking mechanism that is found on most other pistols is not necessary for the .380 because of the round's low breech pressure when fired; the guide spring is enough to buffer the energy displaced to the slide. This simplifies manufacture of pistols chambered for such a round, generally thereby lowering the cost. It also permits the barrel to be permanently fixed to the frame during firing, which promotes accuracy. There have, however, been a number of locked breech pistols chambered in .380 ACP. There have also been some diminutive sub-machine guns, such as the Ingram MAC-11.
The cartridge is ballistically similar to the slightly hotter 9x18 mm (9 mm Makarov) pistol cartridge developed by the Soviets.
The .380 ACP is compact and light, but short ranged and having marginal stopping power. Even so, it remains a popular self defense cartridge for shooters who want a light weight pistol with manageable recoil. It is slightly less powerful than a standard-pressure .38 Special and uses 9-millimetre (.355 in) bullets. The heaviest bullet that can be safely loaded into the .380 is 115 grains (Expression error: Unexpected < operator.
, though the standard has long been 85, 90 or 95 grains (5.5, 5.8 or 6.2 g). Many consider the .380 ACP to be the minimum cartridge suitable for self defense, while others draw the line at the slightly more powerful .38 Special or at the less powerful .32 ACP.
The .380 has had somewhat of a recent upsurge in popularity due to some very compact and lightweight pistols chambered for it, known as Mouseguns.