I carried a 300WinMag on my first grizzly hunt and in retrospect I wouldn't carry anything LESS in the future. So to me, the 6.5x55 would not be a first choice. Here's my opinion on this: If you spend the kind of money it takes to buy a grizz hunt, then one needs to take the time to learn how to properly handle a cartridge with enough energy to safely stop what you're hunting.
