Why is 12 inches considered the magic number for BULLET penetration when 3 inches is the magic number for KNIFE penetration? If a knife can stab an organ 3 inches in, a bullet can hit the same organ 3 inches in. I was contemplating this because of some flak I'm catching for using #4 shot in my 12 ga for HD. I chose #4 due to less over-penetration into adjacent apt rooms (I don't want to be a liability/statistic/on evening news). In ballistics tests #4 shot penetrates from 6-10 inches. That's enough to put a lot of pellets into the lungs, heart, liver, etc.