A 12 gauge is no good against a grizz- ?
You don't know too much about firearms then my friend- ?
You talking a single shot ?
.458 mag ?
30-06 is plenty if shot placement is correct, if you have to turn and just shoot..... then diff story, but a 12 gauge with slugs or a 30-06 is going to take down pretty much anything on the planet - not everything, but close.... again it's shot placement friend