I've had close brushes with bears on many occassions, and not once did I have a scenario where a gun would have helped.
Now, don't go spoiling a good thread with a dose of reality like that.
I've worked in Interior Alaska off and on for the past 12 years, and have too seldom seen bears to seriously worry much about it. My experience both hunting and working has been that bears tend to just "appear", often VERY close, and usually with no ill intent. Truth is, bears are so fast, often so quiet, and so good at ambushing that a lot of actual attacks are such that the person had, or would have had, little or no time to use a gun. I believe Keith Rogan's case as related on TFL was like that.
I've never been charged, but a number of ppl I know have, and it was always a surprise. One fellow's been bluff charged twice, and on neither occasion did he have time to get his gun into action. One bear came out of alder as he as pulling a boat in to tie it off, chased him backwards onto his back in the boat and left just as quick. His brother sat there in the boat mouth-agape; the whole thing lasted but a few seconds. The other bear, a sow with a couple cubs, waded into the river as they were float fishing downstream, and charged at them as they frantically tried to get the boat ashore. Between oars and poles there were neither free hands nor sufficient time to bring their guns into play. And if they had the time, to do what with their guns? There was no legitimate cause to shoot that sow.
No offense to anyone, but planning on head shots is perhaps the greatest folly of all, as the actual frontal kill zone on a bear is ridiculously small, and nigh on impossible to hit on a charging bear whose head is likely to be swaying or bobbing, jaws "popping", prior to or during a for-real charge. Some serious study of bear anatomy, esp. the skull, should be required of anyone thinks they might need to plug one in self defense, just so that one realizes just what they're up against. Practicing on a tangerine swinging wildly on the end of a rope, as it advances on you at 20+ mph while you pee yourself as you haul a$$ in reverse might just replicate a head shot, if one's practice range permits such things.
If you really need a gun, you're likely to already be on your back in the process of being consumed, or in a fetal position trying to cover your neck and melon. In that case, caliber is not your biggest concern. OTOH, bringing a long bbl revolver into play may just be. Or trying to work a single action with one hand as you fight for your life with the other.