Originally Posted by SRH78
...Work cannot be done without energy, period. Therefore, damage must come from the combination of having enough energy and using that energy effectively. That is a very simple fact. What you are trying to debate and where the real debate lies is just how effectively that energy is used...
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.
Of course, for the bullet to be effective in that way, it must have sufficient mass and momentum. And if it has sufficient momentum, it will have some certain minimum energy as well, since momentum and energy each vary with velocity.
But while momentum is proportional to velocity, energy is proportional to the square of the velocity. Therefore it is possible for a perhaps more effective cartridge, with a larger diameter bullet designed to expand appropriately at the velocity at which it is driven and having a large mass and high sectional density, to have less energy than a cartridge firing a small diameter, light bullet with a considerably lower sectional density but at a higher velocity and thus a higher energy.