Alaskan guides carry 375 H & H. I carried a 338 Magnum, the smallest caliber one would want to have against a Griz. Misnomer: Griz may kill but they don’t eat you, black bear will eat you. 12 ga: Best combo is 3” mag with 000 buck, not a slug. Never aim for head, aim for shoulder to break a major bone and roll the animal. Look at a skull of Griz, All shots except eye will bounce off. Best defense if black bear: fight with anything you can find because you are dinner. Best defense if Griz: lay down and cover the back of your neck and crotch and don’t move or scream no matter what (Griz may nip or just sniff and walk away) When choosing between a bolt action and semi auto: go with bolt action because with a semi your gun will be empty when the bear is actually close enough to hit.
I had an Alaskan Guide Gun in 45-70 with a ultra long eye relief scope. Lever action. I suspect it would have done the job.