Your opinion of the 5.56 as a varmint cartridge "that may or may not quickly stop a determined human aggressor" is an opinion, but it's an incorrect one. There are officers, soldiers and operators around the globe that will attest to its abilities in the hands of someone with proper training and shot placement. A human arm is not enough to keep a modern hollow point/self-defense/LE round from performing as advertised.
The location of maximum wound trauma, where fragments and temporary cavity work in synergy, is located in the arm, not the torso. When the bullet exits the arm and penetrates the torso the wound trauma produced in the torso is mild because the bullet is merely crushing tissue it comes into direct contact with, like a handgun bullet does. I refer you to the 5.56mm wound profiles on slide 16 of this presentation (slide with the 77gr MK262 OTM wound profile on top) -
http://www.dtic.mil/ndia/2008Intl/Roberts.pdf . The wound produced in the torso, after passing though an arm, will be similar to .22 LR, as depicted by the last few inches of the wound track after the bullet expands.
"Proper" shot placement isn't always possible because situations are dynamic and adversaries are moving - unless they stand still and are squared-up like a paper target.
The bullet that struck Jamie Martin Wise, unobstructed, in the mid-back, from about 15 yards, nicked his spine, cut his aorta and damaged his liver. Despite the massive trauma (and energy transfer) there was no physiological reason for him to instantly collapse and he was able to act with volition and shoot two police officers, one fatally, after being hit.
In regard to unobstructed torso hits, note that the diameter of the temporary cavity produced by 5.56 bullets in slide 16 is about 4" (about the same diameter as a slow pitch softball). (I incorrectly stated 6” in my last post.) This is insufficient to reliably produce concussion of the spinal cord, caused when soft tissues are propelled radially outward from the wound track. Larger bullets, with greater mass and momentum, create a larger temporary cavity (6-8+”). The sudden displacement of this large mass of soft tissue can cause spinal bones to forcefully slam into the spinal cord, disrupting and stunning nerves and produce instant flaccid paralysis. The small temporary cavity produced by 5.56 bullets is the reason why it’s a varmint cartridge. Yes, it can produce gory looking wounds and kill, but it’s a marginal cartridge cannot be reliably counted on to drop a determined human adversary like a sack of potatoes in the same manner that a larger bullet does. Like a handgun bullet it must disrupt the nervous system to have instant effect. If it doesn’t then an adversary can continue to perform willful activity until blood loss disrupts brain function. There is no “magic” in the bullet’s velocity, energy transfer, fragmentation, etc.
Look for the FBI and other law enforcement agencies to transition to .300 Blackout in the future, like they did from 9mm MP5s to 5.56 AR platforms. Larger, heavier bullets have more mass to fragment, more momentum to transfer, produce greater wounding effects and more rapid incapacitation.