Al, I have bad news for you. If you try to stop-not kill-just stop a Griz or Brownie with any buckshot, you are for dinner. I have hunted every kind of bear there is except Polar. In 95 I got my lifes dream to hunt big Brown Bear in AK, that is Mister Bear to me and everyone else who has any sense. I got him with a 45/70 at 35 yards, and he was not charging, just standing up to see who was dropping in. He was an incredibly magnificant animal, and I watched him for so long, my guide flipped of his safety and I decided it was time. He dropped from a heart shot and flopped around a little, I was actually a little sad and elated at the same time.
Now I have been around bear all my life and I know this to be absolurely true:
There is a world of difference between dropping a big bear who is just interested in you a little, or not aware of you, and a big bear that has decided you are either good to eat or a threat. That bear is going be coming at you at around 30-35 MPH, and will be humping and swaying back and forth, and the only way you are going to stop him-which is different than killing him-is with a brain or a spinal hit with a large heavy slug. From a frontal attack the chances of disabling him are just about nil as all of his vital bones are protected by masses of bone, gristle and more bone. As is his brain and spine. A heavy 45/70 is minimal, a .458Magnum is better, a shotgun slug will do the job if properly placed, and I have a Browning A-bolt rifled 12 gauge that will do that, a smoothbore with a slug can do if it can hit the spine or brain pan-and penetrate. If you are going to use buckshot(and I have a Browning BPS 10 gauge that shoots 0000 buck) you may kill an attacking bear, but he will probably die sometime after you are being digested.
the AK State Troopers carry shotguns loaded with lots of slugs for bear protection, but their best protection is knowing bears and staying out of the bears way. They also frequently carry heavy caliber rifles (in addition to their M4) in the car.