Handguns for CCW, page 3

Takedown power is a myth. The ability of a pistol round (as opposed to a rifle round) to incapacitate relies on a combination of deep penetration, permanent wound cavity and accurate shot placement. Things like expansion and fragmentation are desired but do not occur reliably enough in real life shootings to be relied on.

Realistically the only way to immediately stop an assailant is to hit them in the head or spinal column. Even with a .22LR this would probably cause immediate target neutralisation. However in real life this is rarely achievable, hence few people advocate a .22LR for self defence. What you need is a bullet that penetrates a minimum of 12 (preferably 14) inches in tissue and that causes a fair sized permanent cavity in order to maximise organ damage. Temporary cavities are over rated in pistol shootings due to the natural elasticity of human tissue which allows the body to absorb the effects of this with minimal damage.

.380ACP has been shown not to penetrate significantly enough to reliably cause significant enough organ penetration. When it does penetrate, the permanent cavity is fairly small due to the relitively small bullet diameter.

9mm is a good penetrator, and regularly punches right through a human target. Its' velocity allows it to cause a large temporary cavity, but as we have seen this is of minimal importance in pistol shootings. The high velocity tends to increase reliability of expansion which in turn increases permanent cavity, but it still does not expand reliably enough to rely on.

.45ACP is slower and heavier. In the majority of loadings it does not penetrate the stated minimum amount to guarantee significant organ damage. However it does create a large permanent cavity which increases damage to the areas that it does hit. The main argument for the .45 is its' much-vaunted takedown power. In reality the .45 only generates the same actual energy transfer as a 10lb weight being dropped onto the target from a height of 1.37 inches. This is not enough to knock a target down by any stretch of the immagination.

.40S&W is a good compromise of both. It is wider than the 9mm and therefore produces a larger permanent wound cavity. It is faster than the .45ACP while still holding on to a decent weight, so it penetrates deeper. Therefore we have both reasonable permanent cavity combined with significant penetration.

Summery
Bullets need deep penetration combined with large permanent wound cavity. Expansion and fragmentation do not occur often enough in real life to be relied upon.

.380ACP does not penetrate enough or cause a significant enough cavity to be relied on.
9mm has good penetration but smaller cavity.
.45ACP has large cavity but poorer penetration.
.40S&W has good penetration and reasonable wound cavity.

My choice therefore would be .40S&W.