You beat me to it in mentioning the 5.56 or .223. I heard that the Federal 45 grain hollow point load works great in home defense situations. I don't currently own a weapon in that chambering, so my primary defense choice would be the shotgun with a handgun on person as a backup. Both buckshot and handgun rounds penetrate a lot of building material, so take that into consideration for your particular situation. There are many handgun rounds designed for close quarters defense, such as frangible loads, Glaser saftey slugs ect. I have not heard a solid track record yet on what kind of fight stoppers they are, while some of standard rounds that might overpenetrate, do have a proven record. There are a lot of comprimises in defensive situations.