Go with 180 gr in a premium bullet. I used a 180 gr fail safe on a buffalo a year ago in Wyoming. One shot just behind the front shoulder & the bull went down. He stood up and started to wobbel, then "plop". I killed a mountain grizzly a few years ago with a bigger gun. I would probably feel ok using a 300WSM & the 180gr fail safe, but bigger is better when the animal can eat you.