The .357 SIG case is basically a modified .40 S&W with a skimpy bottleneck stretched approximately 0.020 inch, so that it will hold the bullet more securely. It headspaces on the mouth, not the shoulder.
Federal's goal for the .357 SIG round was to essentially duplicate .357 Magnum ballistics (125-grain JHP at 1,300 fps) in an autoloading pistol. My favorite handloads listed below achieve that level of velocity, and some even exceed it. Care must be taken when handloading bottleneck pistol cartridges such as the .357 SIG, and particular attention to lubing and sizing cases as well as chamfering case mouths must be paid.
Industry maximum average pressure (MAP) for the .357 SIG is 40,000 psi.