The .380 ACP has been with us for more than 100 years. It’s also known as the .380 Auto, 9 mm Browning Short, 9 mm Corto, 9 mm Kurz or 9×17 mm. Corto is Italian, Kurz is German; both translate to “short” in English. Because of these European names, some mistakenly believe the .380 ACP is a shortened version of the 9 mm and that it originated across the big pond. It may have gained popularity overseas, but the .380 ACP is an American cartridge.
While both the .380 ACP and the 9 mm use the same .355-inch diameter bullet, the .380 ACP case is smaller in diameter. Its rim measures .374 inch and the rim of the 9 mm measures .394 inch. The 9 mm case is also longer by .074 inch and was introduced six years prior to the .380 ACP. To further differentiate the two, the .380 ACP is loaded to an average pressure of 21,500 psi. The 9 mm has a maximum average pressure rating of 35,000 psi and 38,500 psi for +P loads.
The .380 ACP was designed by John Browning for a Colt pistol sometime around 1908. The confusing thing—at least to me—is why Browning didn’t just shorten a 9 mm case? No doubt he had one on his workbench. This is possibly because when shortened, 9 mm case walls would be too thick to accept the .355-inch bullet without bulging. Still, Browning could have gone with the 9 mm exterior case dimensions and just thinned the case walls. He didn’t.
There is a more plausible explanation I stumbled upon while playing with a calculator. While my hypothesis is mere conjecture, I’ve yet to find anyone who can offer convincing evidence to the contrary. A .355-inch diameter bullet is about 78 percent the size of the .451-inch diameter bullet used in the .45 ACP cartridge, which Browning had already developed. If you take all the dimensions on a .45 ACP case and reduce them proportionally, guess what? You end up with a .380 ACP. In essence, a .380 ACP is 78 percent of a .45 ACP.
This makes perfect sense and is similar to what Browning did when developing the M2 machine gun in .50 caliber. The .50 BMG cartridge is very proportional to the .30-’06 Sprg.; a cartridge Browning did not design, but he did subsequently design a machine gun chambered for it.
The .380 ACP has always been considered a personal-defense cartridge. It’s really not suited for anything else. When I was working the street, I knew several officers who carried a .380 ACP as a backup. For the most part, these guns were no smaller or lighter than a Smith & Wesson Chiefs Special revolver. Nor did they offer any advantage in capacity. Probably the most well known models were the Walther PP and PPK made famous by super spook James Bond. The Walthers operated on the blowback principle with a fixed barrel, but the new trend of super compact .380 ACP pistols like the Ruger LCP, Smith & Wesson Bodyguard, SIG P238 and the reintroduced Colt Mustang is to utilize a locked breach.