Interesting that you dropped the adjective "unobstructed" from this explanation-
Originally Posted by chaim:
Umm, the FBI sets 6-9" because it is sufficient to hit vital organs from the side. In a "perfectly static frontal presentation", 9" will be enough to nearly pass through most non-overweight adult males.
Umm, that's if you can guarantee
that you will only ever face an unobstructed, perfectly static frontal presentation if you have to shoot someone in order to protect yourself.
I am not so reckless as to assume something like that.