....you are not going to make a head shot on a charging bear. It just isn't going to happen unless you are Annie Oakley and lucky to boot. The area you need to hit is about the size of a grapefruit and is protected behind a lot of bone. Any thing but a spot on direct shot will likely glance off....especially with a lightweight round.
The bear will be moving way too quickly and the shot is far too difficult. With a heavier round, even if a body shot doesn't instantly kill the bear you will likely put it on the ground by breaking it's shoulders, etc. A head shot will only work if you sneak that round into the brain as a lightweight round just doesn't have the penetration necessary for anything else (the rest of the important stuff if hidden behind 11 inches of bone).
Bears are typically alone, they are not particularly social and I have yet to hear a story about a 'pack of bears' attacking someone. At worst it will be a mother and her cubs.
If you are competent enough to make a head shot with your semiautomatic rifle you would have no problem working a lever action rifle, the pump on a shotgun, or a bolt to reload a more appropriate round should follow up shots be necessary.
I can't help but think that you are being deliberately antagonistic because your common sense isn't.
You might as well be trying to convince a NASCAR Driver that he needs to race your '95 Nissan Sentra ricer because "common sense" makes it very clear that your ground effects adds like 50 horsepower.
http://www.skullsite.co.uk/Brbear/brbear_lat.jpg