Read this FBI report saying this whole “stopping power” and “knock-down ability” in large caliber handguns was a myth. There was also an episode in Mythbusters supporting this. But I have a couple of mouthfuls to say to both:
It’s not true that a human cannot launch a bullet at 1,000 foot-pounds of energy (.44 mag) from the hand. Of course you can. It’s not like you’re shooting yourself at the same time with a similar bullet.
A “knock-down” shot should also knock down the shooter. WHAT? The firearm is heavier and shaped differently from the bullet. You have several points wherein “sticky” collisions occur (the gun’s recoil, the hand and arm’s flexing, shoulder and body muscles slightly yielding.) Body mechanics will direct the remaining recoil energy down one’s rear leg into the ground.
A bullet hitting a human is likewise a sticky collision and not all of the bullet’s kinetic energy will be transferred into the body. But you have instances wherein something like that can really happen.
So why can’t a .45 auto (500 f-lb) knock down a man? In Mythbusters, you have a man with his feet braced firmly on the ground, holding a bullet-proof shield to his body. The 500f-lb shot didn’t knock him down. Well, that was a practiced move. A healthy man can brace himself against the ground strongly enough to push a car uphill. But the same man can be knocked down by a strong slap (50 f-lbs.) or a .22 bullet if he’s not prepared.