Agreed, in a stress situation it would be hard to count shots and hard to be rational even. Thats why we have training to fall back on. It helps us figure out what to do under stress. In my opinion the first two shots you make should be as close to center mass as you can get and if he is still coming at you move up to the head shot and keep shooting until he stops.
The likelihood, while in an engagement of making a clean, incapacitating head shot on a moving target with a small caliber pistol at any range shy of arm length is slim to none for better than 99% of the populus. Advocating such practices is really unrealistic. The area of the head which will provide instant incapacitating effect is really no bigger than your fist; it's the cerebral cortex, or brain stem. There have been a great number of people survive being shot in the head (to include brain), and many have retained a good deal of normal function. But, giving the scenario the benefit of the doubt, we'll count any shot from the brain stem up as a likely show stopper. So you're talking about around the nose area north... call it an 8" target. Now compare if you will, your 8" (50.24sq in), moving (and not simply directly at you, but side to side, up and down with the motion of the body) target to a torso (center mass) of the average man... easily 14"x24" (336sq in). Taking these approximations, you're looking at a target 85% smaller than the torso.
And one of the biggest things to consider; lets say you fire 7 rounds... one hits (this is highly optimistic btw)... you have 6 rounds traveling to an unknown destination. Now, lets make this so much worse for you... the one that hit your target; didn't incapacitate him. You just wasted 6 rounds; given your hit percentage, you'd have likely gotten 5-6 rounds in him were you aiming center-mass. People have been known to fight on after taking 5+ rounds to the chest, but it is VERY unlikely. Now we can change our scenario up a touch... of the 6 rounds that missed the target's head... 3 landed in the chest of a four year old girl walking with her mother and sister, killing her instantly. IS that something you could live with... thinking, maybe if I'd have just aimed for the chest, an area much more likely to hit and stop my rounds? It's happened before. I know I couldn't live with that.
The failure drill was never designed to be a defensive drill in a situation in which the typical citizen would have to employ a weapon against an armed adversary. It wasn't even intended for street cops to use. The time and place for a failure drill is in very limited confines with a very low likelihood of stray bullets causing harm, in an offensive operation, with an inordinate amount of training on failure drills. Somehow it morphed into "the thing to do", yet what many don't understand is that outside of those narrow confines it is one of the least ideal tactics:
-Without the training (I'm not talking a class or two; I'm talking weeks and months of training), you are highly unlikely to be successful.
-Without being very close, you're not likely to be successful.
-Without being on the offensive, you lack mental preparedness and that all important time... meaning you won't be successful.
-Being unsuccessful means you miss your target
-Missing your target means you wasted ammunition
-Wasting ammunition means you wasted time
-A missed target means stray bullets flying through the air
-**All of the above means you are putting undo risk upon not only your own life, but the lives of those around you