I may be moving to Montana soon, where there are known to be large bears. I'm thinking about buying a 10-gauge shotgun for defense against them. My ammunition would most likely be 3" 000 buckshot. Would this be a good weapon/ammunition choice for bears?

Also, if so, I'm looking for a shotgun with these specs, and can't seem to find the right one. If anyone could help me find something like this, I'd appreciate it.

-10-gauge (of course)
-Modified choke or able to switch choke tube without rebarreling.
-Semi automatic is preferred, pump is also good.
-A picatinny rail on the receiver would be nice.
-A muzzle brake is also preferred.

Thanks!
Thank you! I guess a 12 gauge should be sufficient. That expands my range of weapon choices greatly.