Jack 99,
Good question....many of my students come to me confused about the same issue. It is not easily addressed.
Let's start with some common tactical school terminologies:
Standard Response- 2 hits center mass (to the center chest vital area.
Failure Drill- (Named based on the theroy that the standard response failed to stop the subject attacker)..After 2 hits center mass immediately make 1 hit to the head.
Mozambique- 2 hits center mass, go to guard position, evaluate effect, then 1 hit to the head if necessary to stop subject attacker.
Guard Position- Handgun kept in shooting stance, grip, etc., but lowered slightly until muzzle is pointed at the subject attackers hands, so you can see the subject and evaluate condition. Sights are lowered to subjects hand level so you can observe the most dangerous part of the body (usually).
What does all this mean? There is probably just as much chance that a subject shot with 2 hits center mass will have some type of body moment or body response as that they may not!!! meaning half the time after two hits center mass a subject could falter, or step back, or maybe lean. The other half they may not give any indication they have been hit. They may not even realize they have been hit and keep on coming (drugs, body armor or determination have effect normal responses). Different people, different conditions, different results.
If you spend any time performing failure drills properly (Perfect Practice, Makes Perfect), at close street combat distances of 5 yards, it is reasonable to beleive you would be capable of 2 hits center mass, 1 hit to the face in 2.5 seconds (on paper).
It is unlikely that real people, if they acted like paper targets, would be able to make any major body movement response in the 1.0 second or less between your 2nd hit center mass and your 3rd hit to the face. And if they did, it is unlikely that you would recognize it unless the head fell completely out of your field of fire.
Add to this the fact that some real subject attackers do not act as paper, nor the common type of moving targets found on most advanced tactical training courses. Some attackers may stand almost motionless and make a good target for you (but if they do, do you have justification for the shooting) and others may move so quickly and eratic that getting a solid hit will take many rounds to accomplish.
If you are in a shooting and survive, you most likely will face a second shooting where the prosecutor shoots your story full of holes. There is good cause to be concerned and to train to evaluate and stop using deadly physical force the moment you are no longer in danger of serious bodily injury or death.
I teach my students to Train to Defend, Train to Survive, Train to Win. It is a fighting doctrine. Once they are committed to the force, they continue to apply the force until the threat becomes less than a threat. Part of this winning, surviving, defending attitude is training to evaluate situations rapidly and accurately. I call this process "Look, See, Access". Under deadly force stress it is not easy to break from the attack and go to evaluating, but it is a very real skill and need, which will enhance your ability to succeed.
Although many gun fights are settled with less than 5 rounds, it is a real good idea to manage your ammunition. As you mentioned there may be many attackers against you. Your constant Look, See, Access will be necessary to evaluate threats as they appear and priortize your actions accordingly within the means you have on your person.
And last, this is simply not the whole of this issue. It comprises many hours of training and information exchange in my courses. I only encourage you to explore the logic behind any shooting drill or tactic. I'm sure there are others reading this, ready to pick it apart.
Best Regards....
"Train as You Live and Live as You Train"