I would be surprised if it were buck shot. Bird shot, NO WAY.
Then again, were it loaded with slugs, it would kind of remove the defense of worrying about intruders. So, if his defense was he was worried about intruders, he probably had it loaded with Buck Shot. If he was awaiting the bear, it was rifle slugs IMO. In either case, it is a happy ending.
Bird shot at very close range lets say under 20 feet is very deadly could easly turn a bears head into mush.
We live just outside Yosemite NP. My neighbor was having problems with a bear raiding his screened in porch where he kept a freezer. Fish and game told him to use bird shot to drive the bear off and give him a rash to remember. The bear returned in the early AM hours a few days later. He used a 12 ga. with bird-shot on him at a range of about 12 feet. Result: Get out the Backhoe, dig a big hole and bury one dead bear.