When I was in New York City in the 1970’s, the NYPD used double barrel shotguns. Their position at the time was that they never needed more than two rounds.
I don’t think they still use them, and I’m definitely not suggesting you get a double barrel shotgun. Its just that you need to forget all the blazing gunfights you see in the movies – with rare exception, that’s not the real world.
An 870 is an awesome weapon even by today’s standards. If you’re worried about recoil, use lighter loads. If you’re worried about bystanders (assuming they’re not too close), use a smaller pellet such as #4 buck.
Also, racking the action and/or making potential assailants look down the cavernous barrel of that thing can encourage these folks to leave you alone without having to fire a shot – always a good thing!
I don’t know if I’m disagreeing with ARlady or not, but keep in mind that any discharge of a firearm, even as a warning shot, is probably going to be viewed by the courts as justified only if deadly force was justified at the time. This becomes real important if someone is injured by that “warning shot”. Typically, you can’t fire a round just to help someone along. (Admittedly, the views of different states and courts on this may vary.)