I backed a guy up on a bear hunt with my HK-91 in .308 while he was using his trusty 7mm Mag. He fired on a bear and hit it with the first shot, which made the bear stand up and charge us. He fired again, striking it off-center, but it barely stumbled before resuming its' charge. I was holding off shooting until I saw my partner's gun had jammed. He was furiously trying to chamber another round when I decided this was all getting too dangerous for me. I fired once to his chest, causing him to drop onto all fours, with the second shot catching him in the forehead. Unfortunately, that tough skullplate caused the round to ricochet off. The third round broke through, quieting him permanently. He was absolutely massive, making him scary even laying there! I had never seen a bear that big.
I have seen some of the smaller calibers fail against the larger animals, so I guess I tend to err on the side of caution. I like .308 for overall game-taking, but when the game is smaller, 7mm Mag is a nice overall round. .300 Win Mag is or 375 H&R are great if you need larger game, but I'll always trust .308 since seeing it drop that massive charging bear.