There are two types of weapons, edged and impact. A bullet is a just a hybrid that goes really fast. The only two ways to damage tissue is but cutting and crushing it.
When a bullet strikes tissue it has the ability to damage the following system listed in the order that the shooter will likely notice the effect and the immediacy the round would have on the ability of the attacker to continue aggression
Central Nervous System- comprised of the spinal cord and head. Since you will likely be shooting into the front of side of your attacker the head is protected only by being a small target and the spinal cord by the tissue in front of it. Comprising the CNS will look like someone turned off a light switch.
Structural System including bones, muscles, tendons, and ligaments- these targets all over the body. The most effective ones being the elbows, knees, and pelvic girdle. Any damage to the hands will likely render them useless for using a weapon, especially that to thumbs.
Circulatory System- the largest, and easiest to compromise. Even though it may be likely to result in eventual death it may seem as though the attacker has not even been struck.
Everyone is familiar with a "ball tap". You walk by a buddy and ball tap him and his head comes forward, depending on how good your ball tap is the farther he will bend over exposing the top of his head and spinal cord to the front.
Most traditional training has us bring our pistol to COM (usually high chest) before beginning to shoot. Doing this we miss have the COM by bypassing the groin to nipple line. There is no reason not to shoot in this area since the muzzle will pass it coming from our hip anyway (at least in a dynamic situation). So those low rounds hit and have the probability of causing the "ball tap" response. As you continue to fire you have a better chance of accessing the heart and lungs with rounds passing behind the clavicle. As previously described you also have a better chance of hitting the head or spinal cord because it has now become the COM.
The pelvic girdle is super vascular and required for locomotion. By default targeting in the manner described you are increasing your odds of targeting the structural system, Central Nervous System, and Circulatory System, in that order instead of the traditional concentration on the Circulatory System alone.
In other words instead of shooting for the head bring the head down to where you are shooting. Also translates well to open hand combatives, edged weapons, and impact weapons. - George