RE: Glock 31 or 21
I have a 357sig barrel for my 27 (same as 31) and I hate it. Very hard round to reload due to the short bottleneck design. And a dangerous round to reload. The rounds using the 40 case are very sensitive to COL. And when you chamber a 357sig with not that much neck tension, you have problem when it pushes the bullet back.
Only way to reload properly is to get a .40cal Factory Crimp die. Put a 9mm sleeve in the die and crimp as had as you can without crushin the neck or case.
Look at all the info on the 40SW, and i feel its just was good as a 357sig.
Ammo will also be a concern for the 357 sig.