I agree with Allen, shot placement and a reasonable caliber is what's required. Some of my most "exciting" days hunting were when a client shot a good sized Montana grizzly (25 permits a year were given back then) in the butt 30 years ago with a Weatherby magnum. He flinched so bad, I'm amazed he hit him at all. The guide, and I (the hired help, who was along for the adventure and the chance to pack out a spike elk bull on horseback instead of carry it out in quarters on my back) got to go in the thick stuff and finish him. The guide carried a 30-06 with 220g bullets, I carried a 270 with 150g bullets (I had recently upgraded from a .243 so I thought the .270 was "big medicine"). Of course this wasn't a Kodiak grizzly, just one of those little Montana ones. These days I'd want a 375 H&H to go in the thick stuff after bear period (or maybe a nice .470 double rifle).

Chuck