Why Twelve Inches?
RetiredUSNChief said this:
The 12 inches mentioned in most discussions comes from the MINIMUM penetration requirements of some FBI Firearms Training Unit testing done a number of years ago as the result of the 1986 Miami shooting.
This 12 inch minimum was dictated under eight very specific sets of testing circumstances, none of which involved an actual human body as the penetration subject.
The study standards were defined because it was recognized that the most important aspect of ANY firearm when used against another human being is penetration deep enough to reliably reach vital organs.
So, when we look at most human beings, we see that the distance between the front of the chest and the vital organs, or even the spinal column, is usually less than one foot.
So why did they settle on twelve inches? That's not to provide an extra margin for a really big man.
First, your attacker may well have turned sideways.
Not only that, there is a not insignificant chance that you will strike an arm: your bullet will have to penetrate the sleeve, penetrate into the arm, penetrate the ulna or the radius or maybe both, and than exit the arm. By the way, exiting the body takes quite a bit of energy.
Then, your slowed (and possibly deformed) bullet will have to enter the chest after penetrating one or more additional layers of clothing, and drive through to hit and damage something vital.
I hope that helps put things in perspective.
No, none of us wants to be shot by a .380, or a .32, or for that matter, a .22 Short. But we do want to come out of a violent altercation unharmed.
As Walt Sherrill pointed out, there are those who will insist on a large caliber defensive handgun. But when we consider that it is impossible to effect solely by means of marksmanship the kind of "placement" necessary to stop a charging assailant, we realize the importance of very quickly being able to fire a controlled second shot and possible a third, or still more.
So--there are compromises involved.