A gun nut (I use that term fairly loosely, I consider myself in advocation of the right to bear firearms to most extents) I once knew told me that in Sweden (or some other, similar European country) there are rarely, if any, robberies in gas stations, banks, and other assorted establishments.
Why? Because everyone has a gun. Well, not everyone, but its easy enough to get one and carry it in public such that there is too high a chance that someone might blast a criminal if he were to attempt to rob a store.