"Bear" with me on this one cause I don't remember the exact story- Two Canadian deer hunters (maybe elk) had killed their prey, opened their actions and started dressing it when they were attacked and both killed by a bear that wanted their kill. Sure would have been nice to have a pistol on your belt in that instance! Where I camp and fish there are three predators- bears, cougars, and humans. I'm confident I can take care of all three! Pepper spray? Our ancestors would be ashamed! When they ventured out in the woods they had whatever weaponry they could make or carry or sharpen. If an animal or rival tribe struck at them they would strike back to kill! Not wound or make feel bad. As far as calibre is concerned you're not trying for a "one shot stop" (humane kill as in hunting). Pull the trigger, then pull it again until you get the desired results. Kill or be killed! I've said it before- pepper spray is for cops and mail carriers!
Best Regards, J. Parker