I've never been charged by a bear, but what I do believe is true about a gun that will stop a charge is you basically need a panzerfaust. Outside of that, whatever gun you have will not stop a charging bear unless you are able to hit them in the brain or spine. Otherwise, no matter where they're hit, a bear committed to charging and attacking will get to you. You need to be able to switch them off. A bear hit in the heart or lungs might have as much as 10 seconds of consciousness before succumbing to the wound. Think of how much damage they might do in 8 or 10 seconds.