Seriously, how about a .44 magnum or a Webbly - I think they came in calibers above .45.
Had a bud who use to do a lot of wandering around in Alaska tell me 15 years ago or so that the only handguns that had a remote chance of working on a griz were a .41 or .44, and only then if you hit 'em in the hump their spine runs through as they charged. He was very much of the opinion that a shotgun or large caliber rifle made a lot more sense. Since then .454 casull et al have come along, but haul the ammo with you as you'll have a hard time finding it in the outback and make sure the first shot hits as you may not recover from recoil in time to place a second.
Anyone besides me remember the SNL "interview" where a horribly scarred Dan Akroyd shilled a book to Jane Curtain titled "Never Feed Bears Marshmallows from your Mouth?"