In addition to zimhunter's post, animal attitude when shot makes a big difference- at rest, eating, stomach full, alert, excited, frightened etc.
: There is no mathematical formula that works perfectly or well for that matter.
: The KO, Pounds Feet(momentum) theries seem to work better than Ft. Lbs., but still don't work all the time.
: Having a relatively small, lightly framed deer absorb 3,000 pounds of anything and still run 100yards makes the FPE numbers seem ridiculous. The FPE formula means that that particular bullet should move 3,000 pounds 1 foot, yet the deer just dashed off weighting but only 120 pounds. Go figure. Slap it in the same place with a .457" 405gr. Rem or W soft point doing 1,500fps with 1,300FPE at impact and it drops - generally. Nothing seems 100% perfect.
: Animals are just that, living, breathing entities, that don't read ballistics tables.
; However - slap them right, with something big, moving at a reasonable pace, and the die quickly, generally.