Huffman has a post that includes this on the subject of bullet penetration:
Penetration less than 12 inches is too little, and, in the words of two of the participants in the 1987 Wound Ballistics Workshop, "too little penetration will get you killed."
Sounds good. But it reminds me of something in the Ayoob book I read. Can't dig it out right now but it goes something like
Saying "A bullet HAS to penetrated 'X' inches as a minimum" has problems. Say you shoot an attacker with a .45; the bullet penetrates the chest, expands to about .65" and stops just under the skin of the back, stopping the attacker cold. According to the 'must penetrate at least' numbers that bullet is a failure because it 'only' penetrated 10.5 inches.
No deep thoughts right now, just something I was reminded of.