Bears break into locked houses all the time. Its not hard for a bear to pull a door right off it's hinges. I personally have had a bear break in and, although the door wasn't locked, he knew how to slide open the sliding glass door. I would think at close range a FMJ .223 from an AR, although wouldn't expand, would cause some serious bone damage and splintering, possibly enough to cause fatal damage and certainly enough to cause CNS or shoulder damage. If a FMJ .223 can go through a car door it can go through a bear's skull at close range. I dont know about you but I can fire off about a shot a second with a semi auto (reliable) AR. The reason why I chose these guns for this scenario is because each one of them at one point has been confirmed, in an emergency, to stop a brown bear attack.