The .357 Sig cartridge was a team development between Sig Arms and Federal Cartridge based roughly on the .40 S&W case necked down to accept .355-inch bullets. It was designed to outperform the 9mm Luger—which it easily accomplished—while being chambered in pistols of similar size. It was also intended to more or less duplicate .357 Magnum revolver ballistics using 125-grain bullets. It was formally announced to the public in 1994, and while most bottleneck pistol cartridges have historically had limited acceptance, the .357 Sig has been successful.

Handloading the .357 Sig does pose certain challenges. As indicated, bullets should measure .355 inch (or 9mm), and not the implied .357-inch bullets intended for the .38 Special and .357 Magnum revolver cartridges. Many 9mm bullets are not of correct design to offer reliable function in the .357 Sig. For example, those made to NATO standards often feature a short enough bearing surface on the shank that they fail to provide sufficient case neck tension. As a result, bullets can be deep seated when hitting the feed ramp, which can result in dangerous pressures. Furthermore, they can cause erratic ignition and poor accuracy. As a result, bullet substitutions are not recommended unless they are designed specifically for the .357 Sig, or are of correct design to accommodate the cartridge’s short neck.

Starline cases were used to develop the accompanying data and provided pleasing results. Although the .357 Sig is based on a necked down .40 S&W case, this practice is not advised, as emerging cases will be significantly shorter than industry specification. As a result, ignition reliability will suffer, as will accuracy.

Cases should be full-length sized to assure reliable chambering. RCBS dies were used to develop the accompanying data. In spite of the .357 Sig being a bottleneck cartridge it headspaces on the case mouth. Bullets should therefore be taper-crimped to industry specifications (.381 inch), preferably as a separate step after the bullet is seated to the correct depth.

Industry maximum average pressure is established at 40,000 psi. Standard Winchester Small Pistol primers were used herein. Upon being fired they showed a significantly flattened appearance, which is normal, as the factory loads tested displayed the same pressure signs in the Glock Model 31 test pistol.

To view the accompanying data, CLICK HERE.