The.357 Smith & Wesson Magnum,.357 Magnum, or9×33mmR as it is known in unofficial metric designation, is a smokeless powder cartridge with a .357-inch (9.07 mm) bullet diameter. It is based upon Smith & Wesson's earlier .38 Special cartridge. The .357 Magnum cartridge was introduced in 1935, and its use has since become widespread. This cartridge started the "Magnum era" of handgun ammunition. The .357 Magnum cartridge is notable for its highly effective terminal ballistics.