Just guessing, The case is under pressure and gripping the inside walls of the chamber while the locking plate (I don't know what the real name is) supporting the head of the case flexes just enough to stretch the case tell it breaks. I don't have a Browning 92 but if it's like a old Winchester 94 it isn't the strongest action out there. Marlin lever actions have been bragging about how much stronger they are over Winchesters for ever just for this same reason. Operating pressure on a 30-30 is 42,000 and 357 is 35,000. My book puts your loads a little hotter than 35,000 psi. Just some thought.