The .357 SIG cartridge was introduced in 1994, as a combination of efforts from the Swiss-German company SIG SAUER and the American munitions manufacturing company Federal Premium Ammunitions. With a rimless, bottlenecked case, the .357 SIG was made to mimic the velocity and stopping power of the .357 Magnum, but was designed to be fired from a semi-automatic pistol.
While the original bullet size was .357 inch, it was quickly phased down to .355 inch for ease of use. It was the first bottleneck commercial handgun cartridge since the early 1960s, with the standard form of the ammunition featuring a bullet weighing in at 125 grain (gr), the same average weight of a .357 Mag bullet. This bullet reaches an average velocity of 1,350 feet per second (fps) and a muzzle energy of over 500 foot pound force (ft·lbs), making it easily comparable to the standard .357 Magnum ammunition.
Although many internet sources say the .357 SIG casing is a sized-down .40 S&W case, that’s not quite accurate. The case of the .357 SIG is slightly longer (.009 inch) than the .40 S&W and thicker throughout, making it more similar to a necked down 10mm.