Unfortunately there's been another officer involved shooting in St. Louis and now I'm seeing a lot of questions about why the officer didn't "just shoot him in the leg?" So it seemed to me that this would be a good time to write a quick post about combat effective shooting.
A lot of people who don't know much about guns wrongly assume that bullets are magic and kill on contact. Many, many people survive being shot multiple times by a handgun. There are stories of people who resisted arrest taking over five shots from a .45 (one of the larger, if slower, handgun rounds traditionally known for its stopping power!) and continuing to fight! Because of this, when people are trained to use a handgun defensively they are often taught about combat effective shooting.
*Disclaimer, I wrote this in 20 minutes. This is intended as a quick overview and starting point*
What I mean by combat effective shooting is simply this: A bullet fired at a threat is effective if and only if it manages to end the threat. There are many ways a bullet can achieve this, the first being psychological. Most people don't enjoy being shot (ya think!?!) and so will cease their aggressive actions once shot. This is the most common result of a defensive shooting, but it is also the least reliable. The bullet hasn't physically disabled the attacker in this case, the attacker has only chosen to stop, something a determined attacker won't do.
The second way a bullet can end a fight is to cause the attacker to loose enough blood to bring about unconsciousness. This is more reliable, but it takes time. Even a shot that destroys the heart leaves the attacker a full 9 seconds of consciousness with which to act. Nine seconds can get you killed in a self-defense situation.
The final way a bullet can end a fight is to damage or destroy the central nervous system. This means a shot to the brain stem or the spinal column. Though damaging those areas is an extremely reliable way of stopping a threat, they are also extremely small and hard to hit targets. Even more so if the attacker is moving.
This means that most people who are trained to use a gun defensively, are trained to make center of mass shots on the torso. Why? This is the best area to damage to bring about enough blood loss to end the threat! Its also a relatively larger target and doesn't move around as much as the head or the limbs.
With that explained, let's look at some specific objections:
"Why didn't he shoot for the leg?"
Because the leg of a running aggressor is an extremely hard to hit target. Furthermore, its not necessarily safer for the aggressor to get shot in the leg. A shot to the knee means he'll probably never walk again. A shot to the thigh could easily damage one of the largest arteries in the human body, which almost certainly means death. It would be better to be shot in a lung than in the thigh!
"Why so many shots?"
Because most defensive gun uses take place at short range there is very little time to incapacitate the threat. This means defenders must place as many rounds on target in as short a period of time as possible. That means a defender will fire two to three shots, or more, before assessing their effectiveness. Most people are trained to shoot until the threat drops.
Its also worth noting that a defensive gun use usually takes a remarkably short amount of time. The rule of thumb is three seconds. Three seconds is not a lot of time for decision making, so people tend to act on training and instinct.
For a more detailed look at handgun stopping power and defensive shooting, see this series: http://bayourenaissanceman.blogspot.com/2011/04/myth-of-handgun-stopping-power-part-3.html