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.[4] Cartridges that are too short can result in malfunctions which may cause serious injury. Furthermore, the SAAMI limit is lower for the .40 cartridge at 35,000 PSI, than the 40,000 PSI for the .357 SIG.
While some people 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 10 mm 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 at low cost from several well-known vendors.
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, so flat-point bullets are used. Various 9 mm 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.
