The .357 SIG (9x21mm) pistol cartridge is the product of Swiss 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 .355 inch bullets, the .357 SIG brass is longer.
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.
Until the .357 SIG, there was no practical semi-automatic pistol round with the same performance as the 125 gr .357 Magnum revolver bullet. There were some semi-automatic pistols chambered for .357 Magnum, e.g., the Desert Eagle and Coonan, but due to the difficulty of designing a semi-automatic pistol able to feed the rimmed .357 Magnum cartridge reliably, they were of an impractical size and price.
The .357 SIG provided a self-defense cartridge close in performance to a 125 gr .357 Magnum, but from a smaller and more concealable semi-automatic pistol.
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 Corbon (1996), necking the .45 ACP down to .40 caliber; the .25 NAA (1999), necking the .32 ACP down to .25 caliber; and the .32 NAA (2002), necking the .380 ACP down to .32 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, though there are exceptions like the .357 SIG chambered Sig 239. .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
The goal of the .357 SIG project was to offer at least the level of performance of lighter .357 Magnum loads and +P/+P+ 9x19mm Parabellum loads. The .357 SIG accomplishes this goal with a 125 grain (8.1 g) bullet. Using heavier bullets, however, shows the cartridge to be somewhat inferior to the original Magnum. The recoil of the .357 SIG cartridge is strong, often noticeably more so than the .40 S&W, but not so much as full-power 10 mm Auto loads or the original .357 Magnum.
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 recoil and ballistics would be identical to the less-powerful 9 mm cartridge.
Because the .357 SIG fires at relatively high pressures, muzzle flash and noise are significant with standard loads, even with longer barrels. Utilizing loads with specialized powders and experimenting with different bullet weights can reduce flash.
Because of its relatively high velocity for a handgun round, the .357 SIG has a very flat trajectory, extending the effective range. However, it does not quite reach the performance of the .357 Magnum with bullets heavier than 125 grains (8.1 g), with the same usable barrel lengths, the typical commercial loadings using 125-grain (8.1 g) bullets, fired from a four-inch (102 mm) barrel; a typical commercial .357 Magnum load propels a 125-grain (8.1 g) bullet to 1,450 ft/s (440 m/s), while a typical .357 SIG load propels the same bullet to 1,350 ft/s (410 m/s), with only a usable 2.85-inch (72 mm) barrel. Specialty loads, such as Double Tap Ammunition, are able to propel a 125-grain (8.1 g) bullet to 1,450 ft/s (440 m/s) from a four-inch (102 mm) barrel. Offsetting this general slight disadvantage in performance is the fact that semi-automatic pistols tend to carry considerably more ammunition than revolvers.
However, this comparatively high velocity can also create the potential for overpenetration. The .357 SIG, much like the .357 Magnum and the similarly necked 7.62x25mm Tokarev, is well-suited for use with bullets that are designed to defeat body armor. Also like the Tokarev, the .357 SIG works well when shooting through barriers. There has been a documented case in Texas where a police officer's .45 round did not penetrate a tractor-trailer's shell, but a .357 SIG round from a backup officer's gun did, killing the suspect inside. The round's ability to penetrate barriers is the main reason for its adoption by law enforcement agencies. However, other documented police shootings have confirmed the round's ability to not over penetrate the body, even though ballistic gelatin shows 16 inches of penetration through heavy clothing (125 grain Speer Gold Dot). The Virginia State Police have had several documented officer related shootings involving the .357 Sig, and in every case, not only were the felons stopped instantly with one shot (except one who was shot several times while attempting to murder an officer), the bullet either didn't exit the felon, or was stopped in the clothing upon exiting, proving that even at such high velocities, the round when used with adequate expanding hollowpoints will not over penetrate soft tissue. The same department has also reported that attacking pit bulls have been stopped dead in their tracks by a single shot, whereas the former subsonic 147 grain 9mm duty rounds would require multiple shots to incapacitate the attacking canines.
The reputation that the .357 SIG round had for losing its crimp (allowing for bullet setback) was partially true when the cartridge was new and ammunition manufacturers were just beginning to produce the round. These problems have since been corrected by major manufacturers. As a result, the round now exhibits nominal setback characteristics, similar to other cartridges.
The bottleneck shape of the .357 SIG cartridge makes feeding problems almost non-existent. This is because the bullet is channeled through the larger chamber before being seated entirely as the slide goes into full battery. Flat point bullets are seldom used with other autoloader platforms because of feeding problems; however, such bullets are commonly seen in the .357 SIG chambering and are quite reliable, as are hollow-point bullets.
One disadvantage of the .357 SIG is that it fires a .355" bullet at higher velocities than most bullets of that caliber are designed for. Very few bullets have been designed specifically for the .357 SIG, and .357 Magnum bullets that are designed for the same velocity range cannot be used due to their slightly larger diameter. Because of this, there are fewer ammunition choices in .357 SIG than one might expect for a cartridge using .355" bullets.
Another drawback of the .357 SIG is its often harsh treatment of the pistols that are chambered for it. Many are designed to fire the .40 S&W and are later modified for use with the .357 SIG. Firing regularly at pressure levels beyond what the pistol was designed for accelerates wear.
The "Accurate Powder" reloading manuals claims that it is "without a doubt the most ballistically consistent handgun cartridge we have ever worked with."