pse-archer -
The "357" used in a Glock 31/32/33 is a 357 SIG caliber, NOT the .357 Magnum. They are totally different cartridges, one is for autos and one is for revolvers. So, you can't use a .38 Special in a Glock. Ballistically, the 357 SIG is similar to the .357 Magnum when firing 125gr bullets. If you're still considering a G31 vs. a G21, I have both (well, almost... I have a G32 and a G21) so I'd be happy to offer my two cents.
