The .357 SIG (9×21mm in metric) is a pistol cartridge, designed by the Swiss firearms company SIG-Sauer.
It is based on a .40 S&W case necked down to accept 0.355-inch (9.0 mm) bullets; the .357 SIG brass is slightly longer (0.009-inch (0.23 mm) to 0.020-inch (0.51 mm)). The cartridge is used by a number of law enforcement agencies and has a good reputation for both accuracy and stopping power.
Developed in 1994, the new cartridge was named ".357" to highlight its purpose: to duplicate the performance of 125-grain (8.1 g) .357 Magnum loads fired from 4-inch (100 mm) barreled revolvers in a pistol cartridge. Performance is similar to the 9×23mm Winchester.
The .357 SIG provided a self-defense cartridge close in performance to a 125 gr .357 Magnum, but from a semi-automatic pistol with greater ammunition capacity.
Other than specialized competition cartridges like the 9×25mm Dillon (1988), necking a 10mm Auto case down to a 9mm bullet, the .357 SIG was the first modern bottleneck commercial handgun cartridge since the early 1960s, when Remington introduced the unsuccessful .22 Remington Jet (1961), which necked a .357 Magnum case down to a .22 caliber bullet, and the .221 Remington Fireball (1963). Soon after the .357 SIG, other bottleneck commercial handgun cartridges appeared: the .400 Cor-Bon (1997), necking the .45 ACP down to .40 caliber; the .440 Cor-Bon (1998), necking down the .50 AE to .44 caliber; the .32 NAA (2002), necking the .380 ACP down to .32 caliber; and the .25 NAA (2004), necking the .32 ACP down to .25 caliber.