I would put the minimum grizzly round at the .308 Winchester. A good 180 grain premium bullet will do the job on a broad side shot.

My personal choice would be the .375 H&H magnum. I wouldn't consider that the minimum though, as some people suggested. The .375 with something like a failsafe or triple shock bullet should bust up a grizzly pretty good lol.

I'll let you know, after I shoot one with the new CZ 550 safari I'm getting!