This cartridge is based upon the .40 S&W case, simply necked down with a short neck and a sharp shoulder. The design purpose was to achieve .357 Magnum revolver ballistics from typical semi-automatic pistols. This cartridge design offers several potential advantages.
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First, its compact nature allows use of a smaller (shorter) grip frame in pistols so chambered. Second, compared to the parent cartridge, the .357 SIG can effectively launch lighter bullets at greater velocity to achieve similar muzzle energy with less recoil. The .357 SIG is loaded to a comparatively high pressure level—the same as top factory .357 Magnum loads and 14 percent higher than the .40 S&W or the 9mm Luger.
The combination of high pressure, reasonable case capacity, and no barrel venting (as seen in .357 Magnum revolvers) allows this petite cartridge to generate significant ballistics—fully the equal of the .40 S&W in terms of muzzle energy. However, in the typical short pistol barrels used, there is a price to pay for this level of performance—muzzle blast is significant.