(ie, 500+ ft lbs per shot, mean expansion of the bullet, etc) than one has to go with lots of truly swift tries at the head. This is not as difficult as many think, since the distance is rarely more than 3-4 ft beyond the muzzle of a pistol in outstretched hands.
The brain is a 4" circle, and any hit to the head tends to rock a guy's world. Yes, a pocket .22lr only has 60-70 ft lbs, but a jab to the nose has less, and it still messes up the guy who gets hit with it. With practice and a good SA auto, one can easily get 6 hits per second on a 4" circle, at 6 ft from your feet, 3 ft from the muzzle of a gun held out in both hands. That is to say, .20 seocnd split intervals between hits, with the first shot not being part of the timed-event. That's a lot of "jabs" to the head, in very little time, and one of them is pretty damned likely to "find" an eye socket, temple, or sinus cavity, and thus, be able to penetrate to the brain.
Such ability takes a lot of "coldness" in the mind of the defender, as well as a lot of practice. It's a lot more likely to be done with the 2c per shot .22lr than with 30-40c a shot centerfires, (that nobody reloads for, like .25, .32, .32 mag, etc).
If you can't hit a 4" brain circle at 6 ft from the muzzle (ie, 9 ft toe to toe) then geometry PROVES that you also can't hit the 12" circle of the chest, at more than 6 yds from the muzzle. Since everyone here is SURE that they can reliably hit the chest of an attacker at 25 yds, then they should be quite confident in their ability to hit the head at 9 yds. 9 yds covers over 99% of the justifiable use of lethal force, by civilians.
Indeed, 90+ % of the time, you wont have to fire. 70+% of the time, the attacker has no gun, so you can't justify firing at him at more than 10 ft or so. I've known Dennis Tueller since 1978, where were you? The point is, by the time you DRAW and fire in the Tueller Drill, the guy IS closer to you than 10 ft. Most guys are so slow on the ccw draw that a knife man who charges them from 7 yds away will be all OVER them before they can "clear" the concealing garment and get a good chest hit.
The Dept Of Justice does an Annual Crime Survey. Check it out on their website. On the same site, check out the FBI's Uniform Crime Report. It says that over half of the GUN fights occur at less than 10 ft. Things in a fight happen very fast, and you can easily end up grappling with the attacker for control of YOUR gun, if you are not fast enough in getting the sort of hits that will stop him instantly.
Stopping him 5 seconds after hitting him is very close to being worthless. It's quite easy for him to stab, shoot, or club you 4x per second, with either hand, and he can easily have a friend helping him attack you. Therefore, even a .50 second delay between your hit and him stopping is too slow, in many cases.