“GLOCK 31 is chambered in .357,”
It is chambered in .357 Sig, not regular .357. Regular .357 is a rimmed cartridge which will only work in a revolver. As I mentioned earlier, the .357 Sig is a necked down .40 S&W (10mm) rimless case that holds a .357 dia. bullet.
Check it out:
https://en.wikipedia.org/wiki/.357_SIG
Coonan and Desert Eagle have both produced semi-automatics in .357 Magnum... with varying results.