I wouldn't bother - what if the gun jams or something? I've read that warning shots get more people into trouble than save trouble.
If a bear's worth the trouble of a warning shot, it's worth the trouble of killing. If it's just wandering around and you want to send it on it's way, put a bean bag in the pipe and keep two slugs in the tube - you still have your two kill shots in case it takes offense
What would the Muzzle Velocity of a Slug be out of an 8 1/2" barrel?
