Found this at the Caliber Corner. The article is from www.hipowersandhandguns.com: http://www.hipowersandhandguns.com/9mm vs 45.htm Taken from the article: A bullet's momentum can be thought of as how much "push" or "shove" it has. Though it has been known for decades (at least since the century old Thompson-Legard tests), neither the .45 or any other round tested at that time has enough "shove" to knock a human being down. Indeed, when bullets were shot into corpses suspended by the neck using rope, movement was documented as minimal at best. In other words, the .45 auto will not physically "knock" a man down and neither will a 9mm. If you see claims otherwise, the writer may actually be reported a physical event that he truly thought he observed, when in fact it was caused by the target's nervous system's response to the bullet impact, but it was not from momentum. None of the commonly used defensive handgun calibers pack enough momentum to "knock" any human down.