The .357 SIG pistol cartridge is the product of Swiss-German firearms manufacturer SIG Sauer, in cooperation with the American ammunition manufacturer Federal Cartridge. While 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 such as the United States Highway Patrol and has a good reputation for both accuracy and stopping power.
The United States Secret Service uses .357 SIG in all their pistols.
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 cartridge designed to be used in a semi-automatic pistol with greater ammunition capacity than a revolver. Performance is similar to the 9x23mm Winchester.
Other than specialized competition cartridges like the 9x25mm 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). Winchester also introduced a 25 caliber round based on the 357 in 1960, (256 Winchester Magnum). Soon after the .357 SIG, other bottleneck commercial handgun cartridges appeared: the .400 Corbon (1996), necking the .45 ACP down to .40 caliber; the .440 Corbon (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.
Most .40 S&W pistols can be converted to .357 SIG by replacing the barrel, but sometimes the recoil spring must be changed as well. Pistols with especially strong recoil springs can accept either cartridge with a barrel change. Magazines will freely interchange between the two cartridges in most pistols. .357 SIG barrel kits have allowed this cartridge to gain in popularity among handgun owners. However, the .357 SIG is loaded to higher pressures than the .40 S&W (the C.I.P. and the SAAMI pressure limits for .40 S&W are 225 MPa and 35,000 psi), and may not be suitable for use in all .40 S&W-chambered pistols due to the increase in bolt thrust.
The goal of the .357 SIG project was to offer a level of performance equal to the highly effective 125 grains (8.1 g) .357 Magnum load. The .357 SIG accomplishes this with a 125 grains (8.1 g) bullet at a muzzle velocity of 1,450 feet per second (440 m/s) out of a 4 in (102 mm) barrel, which is generally identical to the velocity achieved by standard factory 125 grains (8.1 g)r .357 Magnum loads out of a 4 in (102 mm) revolver barrel. (A check of advertised ballistics both in articles from the late 1990s & current ballistics tables from ammunition manufacturers show commonly a nominal velocity of 1,350fps for the .357 SIG with a 125gr bullet in a 4" bbl.) The .357 SIG gains extra muzzle velocity when fired from a longer barrel, like an after-market drop-in 6 in (152 mm) barrel.(This might achieve a velocity of 1,450 feet per second (440 m/s)).
With a simplistic approach to physics, recoil being directly proportional to "muzzle velocity x bullet mass" (due to conservation of momentum), the recoil of the .357 SIG is equal to or slightly less than that of the .40 S&W, and less than that of the full-power 10 mm Auto loads or the original .357 Magnum, Handgun Recoil table as well as Federal and. This simple approach to recoil is incomplete since the properties of the bullet alone do not determine the felt recoil, but also the rocket-like blast of propellant gases coming out of the barrel, after the bullet leaves the muzzle. A more accurate view on recoil is that it is proportional to the mass of all ejecta x velocity of ejecta. Even so, recoil calculated in this manner is only the starting point in a comparison with the .357 Magnum cartridge, since the latter is used in a revolver, in which the recoil energy is due to the Bullet and propellant less ejecta escaping between the cylinder and breech, while the .357 SIG cartridge in a semi-automatic pistol using recoil operation, a significant portion of the recoil energy is diverted to cycle the action, effectively prolonging the recoil phase. Of course, other considerations affect the user's perceived recoil, such as the weight of the weapon, front to back balance, moving mass, height difference between the shooter's grip parallel to the barrel, and grip.
In comparing the energy levels of premium self-defense ammunition the muzzle energy of 584 ft·lbf (792 J) of the 125 grains (8.1 g) 1,450 feet per second (440 m/s) .357 SIG load is greater than either the 475 ft·lbf (644 J) generated by a 155 grains (10.0 g) 1,175 feet per second (358 m/s) Speer GoldDot .40 S&W load or the 400 ft·lbf (540 J) generated by a 180 grains (12 g) 985 feet per second (300 m/s) Speer GoldDot .40 S&W load.
Like the 10 mm Auto, the .357 SIG can be down-loaded to reduce recoil to the point where recoil is similar to that of a 9x19mm Parabellum. However, since the .357 SIG uses bullets that are generally the same as those used in the 9 mm Para, downloading it to this point would defeat the purpose of having the SIG cartridge in the first place, as the .357 SIG casing was designed to handle up to 160 gr bullets whereas the less-powerful 9 mm max out at 147 gr.
Because the .357 SIG fires at relatively high pressures, muzzle flash and noise can be significant with standard loads, even with longer barrels. Utilizing loads with specialized powders and various bullet weights might reduce flash.