I have zero experience in grizzly country, but my experience with TSS “buckshot” loads using BB size shot has convinced me that they are one of the best options for stopping an animal at close range.
I can get 49 pellets in a 20 gauge hull. Any one of those will pass all the way through both femurs and the pelvis, or through both shoulder blades and spine, on a full grown pig. The pattern is tight enough that I doubt a bear skull would slip through a shot unscathed at 50 yards. At 25 yards nearly the whole pattern is inside a 2’x2’ square.
A semi auto 12 gauge loaded with BB, T, or #4 buck size TSS in a 3” shell would absolutely blanket a charging bear in pellets