In cartridges as velocity limited as magnum handgun chamberings out of a carbine, there really isn’t a too soft threshold as they always retain enough weight due to the low speeds. Heavy, hard bullets penetrate but that’s it, hence the ‘eat right up to the hole’. That’s an academic way of saying less terminal effect.
Guys have said Grizzlies are different, yes and no. They are no harder to kill at all, in fact they’re identical in what a well placed shot with a good rifle, say a .30-06, does to black bears- even the very biggest of them. I’d love to be able to say I hunted something far tougher and more tenacious as per their reputation, but I found utterly zero difference between a nine hundred pound grizzly and a four hundred pound black bear in ending the show with an appropriate chambering for the task.
What they do differently in their last moments, is they will readily choose fight instead of flight, see the recent Alaska video. This is not a scenario you want a hard cast, slow to act round for, something with modern speed is what you’re after. Again I chose .375, as the best all round balance for stopping a grizzly as it was fast, and delivered .44 mag bullet weight. No down side.
Now again, I like .44 carbines and the like, and will happily carry one fishing on the BC coast in the Grizz as I’m very aware my odds of having to use it are extraordinarily remote when not actively trying to hunt and kill grizzlies. The advice I gave my clients was, pack your elk rifle, for many of us that’s the moose rifle. Even a 6.5 Creed would be handily ahead of a .44 as a bear rifle to put it in perspective. Anything that fits in a normal frame revolver, is a large compromise.