I've no experience "stopping" charging grizzlies, but I have had to stop a couple of blacks in hair-raising circumstances. I've been bluff-charged by grizzlies - but was able to wait them out -without shooting. My Dad, on the other hand, had extensive grizzly experience - killing "about 20" in his day. I was the recipient of many great stories from him on those experiences, and witnessed two of them. So, with that caveat, I'll proceed.
For the most part, my thoughts are pretty much a compilation of the prevailing trends on this thread so far, but with a few twists.
In a perfect "charge" scenario, one central nervous system hit and you'll have a very dead bear, very quickly. One big (7ft black) found that out - and I was only shooting a 100 grain X out of a 25-06 at a range measured in feet, not yards. I know the same thing would have happened with a 9 foot grizzly too. Not the slightest doubt in my mind.
I'd even go so far as to bet the same result would have happened with a 53 grain Barnes X out of my .22 CHeetah too. But as we all know "perfect" CNS hits aren't going to happen every time, hence we need to think about other cartridges.
In less than perfect circumstances (non-CNS hits) it's going to be all about making holes in the animals anatomy. The bigger around the hole is, and the deeper it is - the better. But, and this is a very big "but" - many of the guns that perforate bigger holes in the hide and punch deeper into the bear, kick so much, that people are scared of them, flinch, shoot poorly and even if they do shoot them well - it does take more precious seconds to settle the rifle down, after the recoil of the really big cartridges, for when those oft-needed subsequent shoots might need to be taken. Oftentimes, these really large caliber rifles are also too heavy to be really fast swingers even before they are fired, too.
So for me, it's all about being light enough to carry and swing that you won't be fatigued from packing it, and it'll be light enough to swing quickly - hence to my way of thinking - it won't be a typical .375 H&H weight rifle, and is less likely to be ideal if it's a larger caliber. So, I don't think that bigger is necessarily better. If you've got a guide backing you - go as big as you want to. If you are on your own - think fast and portable.
Because a lot of things can, and often will, go wrong with an angry bear deciding it wants to lay a hurt on a lone man (not with a guide at his side) - having a rifle with more rounds of capacity is aways better. Having a rifle with one cartridge in the chamber and with five more in the magazine is a great reassurance and potentially very useful. Remember - things are very different without a guide at your side.
In this comparison - the 9.3X62 would have it all over the .338 as the "perfect" rifle for this scenario.
It kicks less. It penetrates at least as well. It makes a bigger-around hole. It is available in lighter, more compact rifles. It doesn't loose as much when fired from a short barrel. It carries two extra rounds in the magazine.
I'm sure my Dad would have loved it - that is, if the 30-06 Remington 760 Pump that he eventually settled on, as his own "perfect grizzly rifle" - hadn't been available. With that rifle and his all-steel tip-off Weaver 2 power scope - he thought he was as perfectly armed as possible - for close encounters in the alders with old Ursus Horribilis.
The majority of his grizzlies fell to that rifle shooting 220 grain round nose bullets. He couldn't figure out why anyone would want anything bigger or more potent. He thought, more powder and more bullet than he used, only increased the chances of missing. My dad thought that "elephant guns" (as he called the bigger magnums) were too heavy and too slow to be "perfect" for killing grizzlies in the thick stuff.
"You can't kill them deader than dead" he'd often add with a wink and a smile. Sometimes he'd add - "How much more penetration than "total penetration" - do you need?"
My Dad knew a lot about killing grizzlies.
