First off, I am not a big advocate of anything but buckshot or slugs in a shotgun for defense. That being said, if I lived in an apartment with neighbors just on the other side of thin walls I would use #4 or 6 shot. At distances of 10-30 feet it should stop a threat well enough. I once saw a deer which was shot at appox. 20 feet with a load of #6's (guy thought he had buckshot in just loaded the wrong shell in the darkness of predawn). The deer only went about 20 yards after the shot and when field dressing it we found several pellets which had penetrated the heart and lungs.