I once saw the results of someone being hit in the abdomen with a load of "bird shot" at close range. It's not pretty, as it penetrated well into the chest cavity, and in this case, it was instantly terminal. I've felt that #2 or #4 shot at close range is more than adequately effective and will prevent over-penetration of walls when used inside a house, unless the target is wearing very heavy clothing. In which case I would go for a head shot. But I wouldn't go any smaller than #4 (#4 shot, not #4 Buck).