I have not shot a grizz with a shot gun, but have popped a small blackie at 12-15 feet with a smoothbore Rem 870 12 gauge and a slug. It was a complete pass through quartering away and on a down hill angle. The insides were soup when I found the bear about 30-35 yards away, but it still ran away after jumping about 2 feet in the air.

Good luck with your project.

Has anyone tried using a brass projectile?