I had a conversation about bear defense last weekend with a retired conservation officer. My question was about a 12ga shotgun. I asked what would be the best defense for a bear. He told me that to stop a bear, you need to break bone. What he carried was a 12ga with slugs only. When I asked why no buck shot, he replied about the breaking bone thing. He said that buck shot has its place and is useful in bear defense, but if you want to stop a bear, he preferred slugs.
He also told me about a bear that killed a hiker in Alaska. Him and some police officers were hunting this bear. When they finally killed it and did the autopsy, they found many .45 caliber bullets in it. But the slug was what stopped it because it broke bone. He said a .44/.45 caliber won't break bone and is pretty useless unless you are pumping it full of them and get lucky. And when time matters, emptying a magazine won't solve a pressure situation. A 45-70, I never asked him about.
EDIT:
To the people who have PM'd me asking who it was. His name is Andy and he's the president of the Edson Gun Club.