Here is a link to an article that demonstrates that a short barreled revolver with a magnum level handgun load is effective against grizzly bear:
http://www.thefirearmblog.com/blog/2009/08/31/man-kills-charging-bear-with-454-casull/
This person used an Redhawk Alaskan in .454 Casull. The bear charge was very fast, and close-up and personal. I believe that a short barreled .44 mag revolver loaded with 300+ grain bullets at a minimum velocity of 1000-1100 ft/s is capable of stopping a grizzly bear attack a close range. I also believe that the .45 Long Colt loaded with heavy well constructed bullets at similar minimum velocities are also capable of stopping a grizzly bear attack.
http://handloads.com/articles/default.asp?id=5
I read about another instance in Alaska where an Indian women was out picking berries of some sort with her two small children. A grizzly bear wandered too close to her two children so she apparently walked right up to this grizzly bear and stuck a small Beretta pistol into its ear and emptied the magazine, and killed this bear. While I find this event hard to believe myself, there were other witnesses present that saw her do this. Apparently, grizzly bears, like humans have very thin bone structure at the back of the ear canal.
Bear attacks are generally very sudden, and obviously at close range. Penetration into the brain or other areas of the central nervous system is critical. The small boutique ammunition companies like Garrett, Buffalo Bore, Underwood, Corbon, etc. all load heavy .44 mag, .45 LC, .454 Casull, etc. loads that should be effective enough at close range to potentially stop a bear attack. I also suggest that people look into using the 3 inch magnum 12 gauge Dduplek solid mild steel shotgun slugs that are now being sold here:
http://www.centuryarms.biz/products.asp?cat=120
A short barreled 12 GA shotgun with these solid steel slugs should also prove very effective at stopping grizzly bears at close range. These steel slugs are far more effective than soft lead Foster style slugs at penetrating hard bone like a grizzly's skull.
I will be carrying a short barreled .44 mag, .45 LC, or .454 Casull along with a short barreled 12 GA when I am traveling in Alaska for my own self-defense against close range bear attacks loaded with the proper style, well constructed bullets and slugs.