A .270 will be plenty. I shoot 140 grain noslers and have never had a problem. Alot of people feel that you need the biggest gun possible to shoot a bear and say "oh, you could make a bad shot, then what" Well, if you took the most accurate gun you own (which in my case is the .270) then you reduce the chance of making a bad shot to begin with. As far as knock down power a .270 with the proper load is more than capable. I took a 425 lb boone and crockett boar that didn't even have time to breathe before he hit the ground. Dropped him like a ton of bricks with a 140 grain .270 nossler. In my opinion if you take a huge rifle in the woods because you think your probably going to blow the shot anyway then you might as well stay at home. Take the gun you shoot the best and everything else will fall into place.