You have to consider what your likely threat is and if they have body armor as well, shotguns are very good unless the looters have procured body armor. Have you ever shot any firearm before?
I was afraid this would come up. Body Armor is a nasty thing, because I imagine it would require a head shot or an relatively high-powered rifle shot. In that case, I think I would probably want to have a semi-auto rifle at my disposal. I don't think I can really put the tactics you'd need on here without violating rules, but I think we all know the only real vulnerable place for folks with body armor.
On the other hand, most body armor consists of trauma plating. I am not sure how effective it would be, but I would imagine that getting hit with a rifled slug at one of those plates would still do a fair amount of damage. I would imagine that the force would be enough to knock a good sized person down, and potentially more than that.