One of the biggest problems with bears, is that they are big and tough.  The handguns most people carry will truly just piss a bear off, 9mm, .40, .45, none are big enough to stop a bear unless you are incredibly lucky.  Something larger, the problem is getting your follow on shots on target.  In rifle calibers, most hunters that I know, use bolt action rifles, so you might get 1 or at most 2 shots at a charging bear, and even with a .30 cal round (30-30, 30-06, .308), you may or may not stop the bear in the first shot.  One of the articles pointed out that it took an average of 4 shots to stop a bear, unless you are incredibly lucky or very very good, you aren't going to get the chance at 4 shots.

You would have a better chance against a black bear than against a grizzly, just because black bears tend to be smaller and less aggressive.  That being said, the right tool is bear spray.