Ammunition performance (as I understand it)
Some thoughts based on my webcrawling for data about choosing defensive ammunition.
1. The reason most defensive ammunition performs similarly in tests is that most common defensive ammunition is all designed around the FBI protocol ballistic gelatin test.
2. In general, bullets with a higher sectional density penetrate further. It's not as simple as which bullet is heaviest, as a 147-gr. 9mm bullet tends to penetrate further than a 185-gr. .45 ACP bullet. Sectional density determines how efficiently a bullet can penetrate an object (including air), and it is expressed by the formula SD=M/(CSA), where SD= Sectional Density, M= Mass, and CSA= Cross-sectional area. The sectional density of the 147-gr. 9mm round is .167, whereas the sectional density of a 185-gr. .45 ACP is .131.
The expansion profile of a modern JHP design changes the sectional density of the bullet as it expands. That is, the sectional density of an unexpanded 9mm bullet is .167, but it decreases as the bullet expands, increasing the frontal surface area of the bullet and changing the SD equation calculation.
The calculation for frontal surface area of a bullet as it expands is much more complex, because modern JHPs do not expand in a perfect circular fashion but rather have 'petals' and jagged edges. I think this may be why solid copper bullets penetrate deeper than you would otherwise expect for their weight- the Barnes DPX bullet, when expanded, does not have the lead 'mushroom' of a traditional JHP that greatly increases frontal surface area of the expanding/expanded bullet, and the DPX retains most of its mass at the same diameter of the original bullet, with the expanding petals matching the overall diameter of a traditional JHP, but not the frontal surface area.
I do not have the math to calculate the effect of the shape of the bullet from the front. I.e. I think that a bullet with sharp edges toward the front will penetrate deeper than one that is flattened without sharp edges. This would be for the same reason that it's easy to stab yourself with a nail, but not with a penny, even if the mass and velocity are the same.
3. In general, for two bullets of the same design, the faster bullet will expand to a greater diameter in less time than the slower bullet.
This means that a faster bullet's sectional density will decrease faster than a slower bullet of the same design and weight. Extrapolating, I think that firing the same ammunition out of a short-barrel firearm is likely to produce slightly greater penetration and slightly less expansion than it would out of a service sidearm. I believe this is born out to a certain extent by anecdotal data until you start having serious differences in barrel length, i.e. pistol-caliber carbine v. pistol.
4. Relying on muzzle energy as a gauge of your defensive ammunition's effectiveness is inaccurate.
KE= Mass x Velocity^2, so faster bullets will have outsized kinetic energy compared to slower, heavier bullets, but the performance is likely not going to be all that different- again, a slight edge to the fast bullet in expansion, and a slight edge to the slow bullet in penetration.
Additionally, KE only matters insomuch as that energy is delivered to the target. The only way to ensure total delivery of the energy is to have the bullet stop inside the target.
Even if kinetic energy were actually important, to put it in perspective, the kinetic energy of a .44 magnum 240-gr. Hydra-Shok at 1380 fps from a 6" barrel is 1015 ft/lbs, or 1380 joules. To put that number in perspective, the energy of solar radiation hitting a square meter of earth over one second is about 1000 joules. Another way to look at is that the energy from a .44 magnum is about sufficient to heat half a bottle of water by less than two degrees Celsius.