Just sharing a little knowledge here.
And these comments are in NO way based on my own experience ... I have never been attacked by a grizzly and hope it will never happen :-) So the advice below comes from another forum where there are experienced bear hunters.

If you do decide to shoot the bear, you need a rifle which at least has a 30-caliber and a bullet speed of roughly 3,000 ft/sec. For example, something like a 30-06 or better still a .300 Winchester Magnum. A little bit less bullet speed is OK (the 30-06 is more like 2800-2900 ft/sec) ... I'm just giving you the right kind of range. This comment comes for a very experienced hunter who has shot large brown bears. So this person speaks from plenty of experience. That kind of bullet caliber and speed will penetrate the skull and spine of a large attacking bear.

It goes without saying ... a large bear like a grizzly poses the most serious kind of risk. There are definitely stories of these bears attackign with their heads down, moving low to the ground, and running very fast. It would be a tough job to control your nerves, keep proper aim on the attacking bear, and get off a successful shot.

I'm not saying other guns would always fail. Just passing on what you would really need to have a good chance at success. Hopefully noone here ever winds up in that situation.

Pete2