Something to keep in mind is the difference between the application of long guns versus hand guns in bear defense, and the most likely requirements to deter an attack.
First, heavy caliber rifles and shotguns with slugs give you the ability to compromise a bear's mobility by hitting shoulders, and other structures, where handguns are more of a finesse instrument. If you look at the statistics, handguns, even in common calibers, are as or slightly more effective in stopping a bear attack than long guns are. The reasons for this are two fold -- we don't have to always kill the bear to make it go away, and handguns are handier in hand to hand fighting.
The reason I advocate for a warning shot, is in many instances that makes the bear go away. It has for me with a warning shot fired from a rifle and a hand gun. If the warning shot is ineffective, a shot to the body might convince the bear to leave, and the attack is over. What we all obsess over, and it is fair, is how do we stop the bear that keeps attacking. Since the most reliable way to stop a close attack with long gun or hand gun is the brain, we want something capable of penetrating the skull of a grizzly.