For pistol, this is exactly what the 500 S&W was designed for. They kept making handguns for this bigger and bigger because they kept finding that they needed to. Even the 500 isn't great for stopping a big Bear. It's about the equivalent of a 308 Win rifle round with maxed loads. The problem is that anything bigger is simply impractical to try and shoot. These things have a vicious recoil.
If you're going to carry a long gun one of the Marin Guide Guns is just about perfect. 45-70 is the classic, but I think they make them in 450 and 444, too. Any of those will sort out a big bear at close range nicely. They are also reasonably light and fast.
If your standard is to discourage a less than completely determined bear, the S&W 500 will do the job. If you want to have a high likelihood of stopping a bear that has every intention of killing you and is in a full charge at close range, go with the Guide Gun.
I'm glad you recomended the Guide Gun in .45-70 @Bear44, I bought one not long after Marlin came out with it with the hopes of getting up to Alaska someday and to hog hunt with. Still haven't got to Alaska yet but using irons and propped against a tree it drilled a 20 oz water bottle on the very first shot at around 125yds using Rem. 300gr JHP's. It's light and accurate, recoil is a tad stout with the Rem. round but not too bad. I love mine and can't wait till I get the chance to use it.
