Shot placement or where to aim your firearm at an attacker is the most important fundamental to know and practice next to choosing to carry a firearm for protection. If you're aiming at the wrong place on a moving target you're either going to miss your target or if you're lucky enough to hit the attacker it won't be in a spot that will do enough damage to the attacker to stop them (which is hard enough anyway) or even slow them down. 
     The first thing we must learn to do is shoot Center of Mass (COM). Shooting COM means we shoot at the center of the biggest or most massive point we can see at that moment. If we see a full front view of our attacker the center of mass would be the center of the chest (thoracic cavity), that flat hard spot we call the sternum. If we only see their head then we shoot at the center of whatever part we can see. If they are turned sideways to us the the largest most center part of their side, etc, etc.
     If our COM at the time is the center of the chest or what is sometimes called the thoracic cavity. The are several vital organs in the thoracic cavity that we would consider vital hits but hitting them even multiple times don't guarantee it will actually stop them. The heart, both lungs, aorta, and vena cava are the main ones that would be considered vital hits. The aorta is a major artery and vena cava a major vein, both of which either supply blood to the heart or the other organs. Even with these "vital" hits it doesn't guarantee the attacker will stop. There have been many attackers or criminals shot in these vital organs many times and it didn't seem to effect them. The main reason for this is that they are hopped up on some kind of drug that these shots that would normally put someone down seems to have NO effect on them at all. Well, guess what. Most, not all but a very high percentage of violent attackers, especially the ones that don't know you are hopped up on some kind of drug. With this in mind there are only 3 hits to the human body that are guaranteed to stop the attack. We call these "stop shots". In the chest or thoracic cavity is the spine. A direct hit to the spine anywhere above the hips with penetration of the spine will cause paraplegia (no leg movement) which will cause them to drop. Any hit to the spine right below the level of the collar bone it the C5 disk will most likely cause quadriplegia (no leg or arm movement). 
     With this hit caliber is of no importance whatsoever. It doesn't matter if you're using a .22 short all the way to a .50 caliber a direct penetrating hit that damages the spine will stop them instantly. This can happen in 1 shot to many shots with any caliber. 
     The next stop shot it the head. The head has the brain and the very top of the spine. A direct penetrating head hit is another that has a guaranteed stop. The brain controlling the whole body and the spine sends all the signals to the body. Again, caliber is of no importance whatsoever. A police officer recently got into a shoot out with an armed suspect. The officer had a .45 semi-auto handgun, during the engagement hit shot 34 or so rounds hitting the gunman 14 times, 13 of them were "vital hits", heart, both lungs, and other organs in the body. He was shooting center of mass and hitting his target with about half of his shots. With the "vital" hits the gunman should have stopped but, being hopped up on something didn't. Your normal human body has 6-14 seconds to bleed out but do you have 6-14 seconds to wait while being shot at or attacked for them to loose enough blood to stop. This is the average human body, not drugged up one. The police officer ended up on the ground behind his car, the gunman looked under the car and the officer shot him 3 times in the head without hesitating. The gunman instantly stopped. 
     The point of the officer shooting the gunman in the head, the head was the center of mass. This would lead us to believe that is they don't stop with a COM chest shot we should move up to the head, this has been taught and trained by many and is still taught and trained by many today. This is a very difficult shot on a moving attacker. The head is bobbing up and down and moving side to side very fast. However, there is still another stop shot location on the body that is an easier shot than the head on an attacker that is moving. 
     Instead of taking the chance of missing the head shot on a moving attacker we go down with our shots. By going down we are doing a couple of things. We keep shooting at the center of the body where the spine is and hoping for the bullet to penetrate through to hit the spine and obtain a true stop shot. Also by going down we go down to the level of the hips. A hip shot is also one of the 3 true stop shots. Again, this hit doesn't require any certain caliber to stop your attacker. A direct hip shot will drop your attacker instantly no matter what caliber you have. Study the picture below:
Defense Training
CWP classes
stop shot placement target     

     The picture shows the spine which goes from head making it the biggest of the true stop shots and a hit anywhere on the spine will at least make the legs immovable and if at the C5 or above the arms as well. The hips are the second largest stop shot and there are two of them. When you aim at your attacker and are going down with your shots you continue to go straight down, don't waiver or go to either side. If they are moving there hips will be rotating side to side so they will be going toward your shot placement anyway. Next is the Head being the smallest target as well as the hardest to hit. Remember any missed shot you're responsible for whatever the bullet hits. There are many law enforcement agencies and some military units that have went to this way of training due to missed shots. Remember once your attacker stops attacking you stop shooting.
     The picture above also shows the shoulder joint. While this will stop the movement of the arm we hit it won't stop the attacker. It may stop them stabbing or shooting us but not there forward motion. A hit here is good as long as it wasn't our intended hit. It's good because it still hit the attacker and it did stop something which may slow them down or make their attack less lethal giving us time to get out of the way.  
     We strive to teach and train in a way that has the least amount of learning curve possible. With this we can help those that don't have the time or money to get to the range as much as others or others ways of training require. We also strive to keep it simple due to physical abilities as well. Firearms for self-defense and training with them are not only used by those who are in good physical condition, are younger, have good physical health, but also those who are older, out of shape, and in bad health. I do agree that we must train as much as possible to keep the edge on our attacker, but the easier we make it the more of an edge we'll have over them.