My son says that the only way to knock down someone with the 9MM is to hit them with them gun.
HA!....that's funny!....er, sort of! :-)
You might want to advise your son that pistols aren't designed to "knock someone down". He might want to carry a baseball bat instead. ;)
IPSC PF = wt * V / 1000
Where: wt=weight in grains, and V=velocity in fps
Major is from 175 up. 200 gr. .45 bullet at 900 feet/second = 180 power factor.
Minor is from 125 to 174.999. 125 gr. 9mm bullet at 1000 feet/second = 125 power factor.
Adding the caliber to the formula gives us a yard stick for it’s defensive potential. This formula favors bigger calibers, and heavier bullets... But then again - IPSC's formula was weight friendly too and people can moan about hydrostatic shock but that fact is bigger caliber bullets make bigger holes.