Although I prefer a handgun (which can be used for defense at home and away), Dan Carlson certainly makes a good point:
For home defense you can’t beat a 12-gauge shotgun, preferably a compact model you could move from room to room with if you had to. As to the kind of shotgun, there are two schools of thought. One is that the best choice is a pump action gun. They are reliable, easy to use, and many a would be burglar has fled a home with wet pants after hearing the distinct “click-clack” of a homeowner pumping a round into the chamber. Others say a side-by-side double barrel model has such a devastating psychological impact when it’s pointing in your face that most criminals will surrender as soon as they find themselves staring down those two big tubes of doom. Another reason that a shotgun is preferable is that, when loaded with #4 or #6 shot, it’s unlikely to seriously injure a family member in another room. The sheetrock will take most of the blast and what does get through the other side is likely to be moving at non-lethal velocity.