I'll summarize what I have found from reading threads like this in many forums. Skip the buckshot and use slugs. Of the slugs, the most commonly mentioned are the Brenneke Black Magic Magnum or Classic Magnum. Occasionally someone mentions the law enforcement Special Forces slug from Brenneke as well. Another possibility is the DDuplek's
Steelhead slugs.
An armorer for an Alaskan Fish and Game preferred the Brenneke Classic Magnum, and that is what his agency used at the time. He had an incident where a 3" shell got hung up during ejection and since then relied on the 2.75" shells. He also reported having several DLP kills using the Classic Magnum shells. If you want to read about his experiences on the Outdoors Directory forums over the years (username: stevelyn),
here is a search.
Finally, many who live in bear country prefer a rifle over a shotgun, but shotguns have been used effectively to kill bears, even big brown bears.