A lot of calibers just bounce off grizzly skulls. .44 Magnum or .50 AE (which IS a Magnum round, I believe; I think it's .50 Magnum redesigned for an autopistol) are both better at penetrating the grizzly's skull than the .45 ACP or .45 GAP, but you still have to watch out for the skull deflecting some shots.
Friend of mine carries a 44/70 with a nine inch barrel when he hikes. Still feels nervous. When I get the urge to hike, I lie down until it goes away.