We have the PF due to the personal prejudices of the pioneers of practical shooting. Jeff Cooper and others felt that energy was irrelevant, that "momentum" was more accurate of "stopping power".
Who dares to argue with these gurus is shouted down in a cacophony of attacks on "eggheaded physicists"!
But a rule must draw an arbitrary line somewhere and so we have the PF rule. It is simple to compute, and easily measured.
(So is the Army Physical Fitness Test)
Neither the APFT nor the PF reflect the true capability for performance of the assigned task(s).
Just my $.02 worth!
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Ni ellegimit carborundum esse!
Yours In Marksmanship
http://www.1bigred.com/distinguished
michael