My #1 line of defense against bears here in Kodiak is a good 9oz can of bear spray. I also carry my G20SF in 10mm (200gr WFNGC Beartooth from DoubleTap). I draw them simultaneously, (had a few bears get ornery around me so far) put a shot of OC straight to it's face and then spray a 'barrier of OC' between me and it, slowly backing the whole time. If it shows any more aggression toward me, I'll put as many rounds into it's shoulders and chest (if visible) as I can. I doubt any handgun (with the possible exception of the .460 or .550 S&W Mags) will penetrate a bear's skull and kill it instantly. The brown bears up here have a very thick, hard forehead set at an angle that will deflect most rounds. When measured on a horizontal plane (that is, the horizontal line that the bullet would travel through the skull) the thickness of the forehead is about 4". That's like trying to shoot through 4" of something that's harder than concrete set at about a 45* angle. All that trying to hit a brain the size of a softball? I doubt it. The only chance a head shot has of killing a bear is a shot through the nose (softer bone and a semi-clear passage to the brain) but the angles have to be right for this. A better bet (IMHO) is to shoot and disable the front shoulders, preventing further quick movements and giving you time to take a well aimed shot to the side of the head which is less protected. I don't have any fancy scientific data to back this up, I've just seen the skull of a Kodiak Brownie and it's thick and very well protected at the front. The sides offer a much better chance of a killing shot. And the only way I can think of to get a clear shot at it's temple is to disable further movement first. Killing a charging bear is probably every bit as difficult and dangerous as trying to kill a charging cape buffalo. The very best bet is to try and stay away from such encounters. I.E. pay attention to what's going on around you and get the **** outta there if it looks like there might be trouble. That being said, not all cases are preventable and you should be prepared for the charge, should it occur.
Just my $.02
BTW: I have not had a bear continue aggression after deploying the bear spray.
Hope this helps. Best regards.
JG