The new cartridge was dubbed a "357" in order to highlight its purpose: to duplicate the performance of 125-grain .357 Magnum loads fired from 4-inch barreled revolvers, except in a cartridge designed to be used in a semi-automatic pistol.
Until the .357 SIG, handgun owners looking to match the performance of a 125 gr .357 Magnum bullet in a semi-automatic pistol did not have any practical choices. There were some chambered for .357 Magnum: the Desert Eagle and Coonan, but due to the difficulty of designing a semi-automatic pistol that can reliably feed the rimmed .357 Magnum cartridge, they were impractical because of their size and cost.
Development of the .357 SIG provided access to a self defense cartridge that would come close to the performance of a 125 gr .357 Magnum load from a smaller, and more concealable semi-automatic pistol.
Most .40 S&W pistols can be converted to .357 SIG by replacing the barrel, and sometimes the recoil spring. 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 (a difference of up to 5,000 psi at top loads), 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 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 is not as harsh 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, both muzzle flash and noise are significant with standard loads, even when using longer barrels. Utilizing loads with specialized powders to reduce flash and experimenting with different bullet weights can reduce flash.
Although the .357 SIG design is based on the .40 S&W case, handloaders cannot form .40 S&W cases into .357 SIG brass. While the two cases are identical in rim diameter, using the .40 S&W case will result in a case that is approximately 0.020 in (0.508 mm) too short. Unlike most bottlenecked cartridges, the .357 SIG headspaces on the case mouth; cartridges that are too short can result in serious malfunctions, possibly leading to serious injury. Furthermore, the SAAMI limit for the .40 cartridge is set at 35,000 PSI, but at 40,000 PSI for the .357 SIG.
While some individuals have attempted to form .357 SIG cases from 10 mm Auto cases, this is rather impractical. First of all, the change in primer type (10 mm uses large pistol primers, .357 SIG uses small pistol primers) would require that known recipes be scrapped, and second, the cost of using expensive 10mm brass for such a purpose would certainly defeat the goal of saving money by reloading. Both unfired and once-fired .357 SIG brass are readily available from several well-known vendors, at a low cost.
Choosing the correct bullet type is extremely important when handloading the .357 SIG cartridge. The short neck of the casing makes the use of standard round-nosed bullets impractical. There is simply not enough flat area for the neck to "grip" the bullet. For this reason, flat point bullets are used instead. Various 9mm hollow-point bullets can also be successfully used, but due to the wide variety of choice, the chances of success vary from one brand to another.
Because of its relatively high velocity for a handgun round, the .357 SIG offers a very flat trajectory, which in turn allows the firearms chambered for it more effective range. However, it does not quite reach the performance of the venerable .357 Magnum with bullets heavier than 125 grains, it comes close, with the same usable barrel lengths, the typical commercial loadings using 125 grain bullets (e.g., fired from a four-inch barrel, a typical commercial .357 Magnum load propels a 125-grain bullet to 1450 ft/s, while a typical .357 SIG load propels the same bullet to 1350 ft/s, with only a usable 2.85 inch barrel). Specialty loads, such as Double Tap Ammunition, are able to propel a 125-grain bullet to 1450 ft/s from a four-inch 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.
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 effectively beyond what the pistol was originally engineered for tends to translate to accelerated wear on the firearm.
The "Accurate Powder" reloading manuals claims that it is "without a doubt the most ballistically consistent handgun cartridge we have ever worked with."