I carry a 5" Smith and Wessen Performance Center 8shot .357 mag. full of Buffalo Bore 180gr Hard Cast. I personally don't have any qualms about a .357 with proper ammo on MT grizzlies, though like you said; Spray is the first line of defense.
You mentioned a .454 Casull. That cartridge will only be available in a Revolver, and Realistically, about the only cartridge you will find in a semi-auto that may be stout enough for a griz (IMHO) would be a 10mm Auto. Personally, I don't trust the reliability of revolver cartridges in Semi-auto's and they are prohibitively large for my taste. If you want to stick with a semi-auto... I'd pick up a glock 20, some heavy hard cast ammo, and sport that. Otherwise, Any of the modern revolver cartridges .357 mag or larger will suite you well. .44Mag and .454 Casull are also good options. I would look at revolver cartridges with factory downloads avaliable (.38spl, .44spl, .45LC) for practice and work on the draw. If you go with a revolver, I would not carry a barrel longer than 5". They are quite cumbersome and far slower out of a holster than a traditional semi-auto.
As you've seen, Bear encounters happen very quickly. What ever you get, Practice, Practice, Practice. Were I in your shoes, I'd practice to be able to hit an 8" target at 10 yds from the draw within 2 seconds, with the holster that you are going to use in the field. Use a shot timer, Aps are available for almost all smart phones if you don't have a timer. Is it possible, Yes. Is it easy, Oh **** No. I have made it to about 2.2-2.4 seconds on average with my 5" .357. That's about as fast as I get, but I keep practicing and pushing myself to get faster to the first shot. To give a bit of contrast, I can do the same thing with my Glock 19 in about 1.6-1.7 from open carry and about 2.2 from concealment.
If I had it to do all over again... I'd probably grab a Glock 20 and never look back.
