Excellent information and completely factual. Your preferred target is the same as mine has been in most cases. Thanks for that post.A well thrown rock as well as a pointed stick can can impair and even kill. I am not suggesting that we should carry rocks or sticks for self-defense. I am asserting that any caliber ammunition can be lethal, and therefore can incapacitate a person.
Lethality of any caliber is limited by the skill set of the shooter not just by ballistics. That skill set includes speed, accuracy, and understanding human physiology.
Speed — Their is little time to react if attacked by a near adversary. You have to be able to deter or impair the attack very quickly, maybe in a second or two. This is why self- defense preparation should begin with learning basic unarmed combat skills. For example, if an adversary is coming at you from two feet away with a knife you have to be able a to react in a second or two. If you go for your EDC, you are likely to be stabbed because you are extremely unlikely to draw and get off a shot in a second or even two. You need to parry the attack to gain time to effectively use your gun. Even when the adversary is far enough away that you do not need to parry speed is critical. If an adversary is twenty feet away and pulls a gun, he has an advantage. His gun is usable and yours is holstered. If you cannot draw your gun and have it on target in a couple seconds, you chances of going unscathed decrease. The longer it takes the less chance you have. But if you are fast on the draw you are likely to slow down the adversary by the psychological impact of an unexpected gun pointing at him. If you shoot and hit him, even if a superficial wound, it will slow him down further.
Accuracy — Famed Marine General Chesty Puller remarked to his men: "You can't hurt 'em if you don't hit 'em. Getting shots off quickly and missing is a good way to end up injured or dead. A hail of bullets can distract the adversary without hitting him, but it won't win a gunfight. Only hitting the target is going to lead to victory, and the more accurate your fire the more likely that you will win. That is regardless of caliber. Simple fact is that a 22LR in the face is going to impair or incapacitate your adversary. A 9mm bullet in the arm might not do that.
Human physiology — The lower the caliber the more important a knowledge of human physiology becomes. Knowing the location of critical organs and nerve centers is important to knowing where to aim. Center mass is a very large area of the body. There are some place better to aim for than others. For example, I would aim for the bottom area of the sternum. A hard fist punch to the sternum can incapacitate and even kill a person. If my bullet hits high of the sternum it hits the neck. If it hits center, it can deflate the lungs. If t hits low, it can impair heart function. If the bullet hits right of the adversary's sternum it hits a lung. If it hits farther right, there are major veins. If it hits left, the heart and aorta are exposed. If you shoot high and hit the neck the bullet can reach the spinal column. If you hit below the sternum you hit the solar plexus nerve center of the torso and the liver. The area of the sternum is a rich incapacitation zone.
I consider clothing as an adjunct to physiology. A tee shirt will not slow down a lighter caliber. A tee shits, regular shirt, fleece vest, and winter coat can slow down a bullet, and in lighter calibers that can meaningfully impair the ballistic performance. So clothing can be a factor in selecting point of aim. Anything that will affect penetration must be taken into consideration regardless of caliber. If I saw an adversary was wearing body armor, I would aim my 9mm for the neck or face, but not the forehead.
To the OP's point, I believe that 22LR or 380 ACP can be adequate self-defense rounds provided that you absorb the importance of the above information. When the day comes that I cannot handle 9mm, I will bypass 380 and go to 22LR, because I know that I am and will be more accurate with 22 than 380.
Note: Oddly this thread related to the first post of my blog that will go public on October 10. I decided to publish this as a harbinger of more to come.