The energy transfer into the target is roughly its "stopping power". The goal is to remove an imminent or active threat. Your newtonian physics intuition is cor

The energy transfer into the target is roughly its "stopping power". The goal is to remove an imminent or active threat. Your newtonian physics intuition is correct: lower velocity and/or lower mass is going to lower the energy of the round. If the goal is to stop the individual, you want to be able to deliver as much energy as possible into the target. Lower velocity/lower mass rounds aren't going to be as effective in stopping the target. If a bullet passes through an individual, it hasn't delivered all of its energy to the individual. It's still going, so this is inefficient and dangerous: you're less likely to have stopped the target, and you're more likely to hit something unintended.