The error is to focus on energy. A suitably effective cartridge will have adequate energy. But it is not the energy that is causing the cartridge to be effective; it is not the energy causing the damage (at least at the range of velocities of handgun cartridges customarily used for self defense). It is the combination of factors resulting in a bullet of large enough diameter penetrating deeply enough; the necessary tissue damage is caused by the penetration of the bullet.
Sometimes when trying to explain something we get our feet tangled and trip up, maybe that's what happened above.
Energy does not do the damage either in handguns or rifles, the bullet does the damage in both. Energy has neither weight nor mass so in and of itself it can do no damage. There are no energy beams at present.
We all know that the faster we get a bullet going the more potential damage that bullet can do. It doesn't matter the caliber or the mass as long as the bullet is a good one for the speed it's going and built right for the job the faster one has the potential to do more damage once it strikes.
But why? Simply because what ever it hits can't get out of the way fast enough. The bullet pushes it's way through flesh and bone, tearing and disrupting tissue as it goes. As it expands, providing it does, the amount of disruption increases. At lower velocities the amount of damage is usually limited to the area right around the bullet's path. At higher velocities the damaged area can be several times larger. I've seen wounds where bone has been cracked and a neat hole punched through from a 45 Colt. I've also seen wounds in deer and hogs where sections of flesh were liquified and the bone splintered and fractured from high velocity rifle rounds.
In general the faster we get a round going the more damage it can potentially do. There are other factors involved of course, a .22 l.r. round will not do as much damage as a 38 Spl. even though it is faster.
The energy figures we use, so many foot pounds, etc., measure the potential for the bullet to do it's work. But in and of itself the energy does no damage. The bullet does the damage.
Energy cannot be destroyed only transferred. As the bullet travels down the muzzle, through the air, penetrates, expands, etc. it sheds energy. Transfers it, mostly into heat, the bullet utilizes the energy to do it's work. All bullets do this. A .45 acp at 860 fps does it as much as a 30-06 round at 2800 fps. They both utilize energy to do their work and because one has a good deal more energy than the other it can do it's work over a longer distance and do more damage once it gets there. It has more energy, in this case, because it's faster among other things.
We've known this since we were kids and throwing rocks. We observed all this then. We threw rocks in lakes and puddles, into mud and watched the results. We debated the merits of cinder blocks, vs. bricks, vs. a good hard round stone from a stream, vs. a dirt clod.