Realistically, I think most people will just keep pulling the trigger and not worry about if its in the head or in the chest. My question is why not start with the head? Why move to the head once you have started with the chest? Why not keep going with the chest?
I doubt that most people will make these split second decisions thinking of where the bullet will go to the head or the heart.
1. Most people would not think head or chest, they would just blaze away. (That does not make it right.)
2. You are not most people if you choose so.
The first step to dealing with a possible failure to stop is to really understand that it is entirely possible and have a plan of what to do should it occur. Those who have not done so will certainly fail to adapt to the situation. Those who have considered it seriously and practiced should be able to adapt to the situation. Most people will not do so, then again I believe most people are asleep and most have never seriously considered a lethal force encounter let alone a failure to stop.
Why not continue to throw lead into the chest?
There are two explanations.
1. Simple: As Dale Carnegie said "The definition of insanity is doing the same thing over and over and expecting a different outcome."
If two rounds have failed to stop in the COM how many others do you want to try? How about all of them? If it still hasn't worked then what?
2. Detailed: People fight because they want to and because they can. The only sure stop is to either shut down the Central Nervous System or inflict enough damage that the target bleeds out to the point that he cannot function. (A third is breaking significant parts of the skeletal system to cause him to drop but that is very unlikely with a handgun and the bones needed to be broken.)
If hits to the torso are not stopping him because he is intent on murder and or is so medicated he does not feel them then you can give up on trying to convince him to stop through pain. You best option is to shut down the CNS which is located in his melon. More in the torso MAY result in a spinal chord break and will cause him to bleed out faster but between adrenaline after already taking two and any other factors mental or pharmaceutical he is not going to stop until he runs out of blood and that can take a long time when someone is intent on murder.
If hits to the torso are not stopping him because he has on body armor, which is FAR from unheard of on rampage killers and criminals these days, then more to the torso are going to accomplish nothing.