I think it's more about semantics than anything else.
Yeah, we have all seen a pistol round knock down a bowling pin. It's just about all a pistol round can do- anything much bigger than a bowling pin stands a good chance of absorbing the hit and staying upright.
But going back to the vest wearing man and the nine millimeter...
I have experimented with a "Timed Out" vest before. (They have an expiration date)
After shooting it with everything from a .22lr up to .44mag, I can tell you that it does in fact stop the bullets from penetrating.
I can also tell you that it knocks the crap out of whatever is behind the vest.
I know that it's not scientific, but I tried to duplicate the dent left in a pine board that was behind the vest when it got shot with .45acp.
It turns out that smacking the board with the ball end of a 16 ounce ball peen hammer with everything I can put behind it pretty much makes the same kind of dent in the wood that the .45 leaves behind the vest. Now, I'm not Thor or anything like that, but I don't know anybody that would let me hit them for all I'm worth in the chest with a 16 ounce ball peen hammer.
And that brings me back to knockdown power.
I'm pretty sure that a grown man will in fact end up on the ground right after catching a pistol round in the chest while wearing a vest. The energy transfer is still going to be massive and painful, and it just might turn a man's lights off for a little while. It's absolutely going to knock the wind out of him. Probably break some ribs too.
Now, I realize that the pistol round doesn't have enough power to literally take a man off his feet and throw him down...
But if he hits the ground right after taking the bullet, can't we call that a knock down?
I'm pretty sure that nine out of ten people watching a man "BangFlop" from a pistol round would say he was knocked down.
It's semantics.
http://en.wikipedia.org/wiki/Semantics
If we can't call a man that hits the ground in the same second that a bullet impacts his vest "Knocked Down", then what
DO we call that?
Here's a definition from the online dictionary:
knock down
1. To bring to the ground with a blow; topple.