For what it's worth, I agree that energy is not meaningless.
A bullet wound is a combination of a penetrating wound (like a stab wound) and blunt trauma (like getting hit with a fist, a club or a bat).
A very low energy bullet will emphasize the penetration aspect and there may be very little blunt trauma effect.
A higher-energy bullet will increase the blunt trauma effect.
Fackler correctly noted that the blunt trauma effect (the temporary cavity we see in freeze-frame or slow-motion gel shots) was not a consistent wounding factor in handgun wounds. If there's enough energy and if the bullet hits in the right organ, there can be some wounding effect, but a lot of the human body is pretty stretchy and it snaps back leaving only the equivalent of a bruise. The liver, kidneys, brain, spleen, tend not to stretch and if you get a temporary cavity in one of those organs, the effect can be lethal.
But instead of just leaving it there (saying it's not consistent), he tried to downplay even that effect and ended up swinging too hard towards the idea that in handguns, the ONLY wounding effect was the permanent wound channel.
That neglects the organs that don't stretch without tearing, but there's also more to stopping someone than just wounding them.
A penetrating injury may not be immediately obvious to a person who is under a lot of stress. It's not uncommon for stabbing victims to comment that they didn't realize they had been stabbed until they saw the knife, or saw the wound, or noted the bleeding.
Blunt trauma is a different story. When a person is slapped, they aren't likely to sustain an injury, but they will certainly know immediately that they have been slapped. You don't hear people say things like--"I didn't realize I had been slapped until I saw the red marks in the mirror." or "I didn't know he hit me with the bat until I saw the bruise." Blunt trauma of any significance is noticed immediately.
So while the penetration aspect of a bullet wound may be what puts someone in a hospital or in the ground, the blunt trauma effect (if it's significant) is likely what lets them know they've actually been shot. That's really important because one of the most significant factors in whether a person stops attacking/stops fighting is psychological. If a person doesn't know they've been shot, they are more likely to keep attacking, to keep fighting whereas if they realize they've been shot, they are likely to become more concerned with their wellbeing than with continuing an attack or continuing to fight.
IMO, these two things, the notification effect of the temporary cavity, and the injury in non-elastic tissue, are the two reasons that energy does matter, even in handguns. How much it matters depends on a lot of things.
