It's all about 2 factors. Shot Placement, and Trauma. 99% of the time, if you shoot the BG in the right place, that's the end...HOWEVER... add in stress, adrenaline, fear, xcitement, and all of the other little factors, coupled with the reluctance of many agencies to properly train, and you see why our hit ratio in combat (LEO's) is as statistically dismal as it is.
TRauma. Small, light High-velocity Bullet V Big, Heavy, Slow bullet.. Big, heavy and Slow Wins IMO. the idea is to disrupt his system as much as possible to shut him down NOW. IMO this is best done by punching Large, Messy, open holes to let his insides become Outsides, and to distract him from attempting to do the same to my own tender, pink (and rather cute ) dermis. People say that "Stopping Power" is a Myth..and if you look at in the way it has been protrayed by some, they're right, it is..or is it, if you shoott he BG with one (or even 2 or 3) 9mm's and DONT hit him in an instantly fatal spot, will he still be able to bring the fight to you. Conversely if you do the same with a couple of .45 ACp's, that are going to tear him up, and boost him into shock more quickly, will that be more likely to "Stop" him from engaging further??