Originally posted by searcher
Not trying to be a physics geek here but Newton's laws do allow for a small body in motion to knock down a larger object it hits, even if an equally sized object that was free to topple over (but didn't) was holding the "particle projector" that imparted the motion to the projectile. The key is the time it takes the projectile to "dump" (transfer) its energy to the target (described in physics by the term "impulse")in a collision. The other thing that influences this is the "hardness" of the projectile and the target (coefficient of restitution). Intuitively this makes sense with the example of catching a fast moving ball. If we catch the ball by moving it back while we catch it (increasing the "catching time") the force we feel will be less than if we catch it with a stiff hand(stings like hell!). This is the same thing that lets a martial artist break bricks with his bare hand, even though a man can lift much less weight on a bench press than it would take to collapse a brick designed to hold the weight of buildings.
So all you need to do is shoot a guy holding a thick glass block (glass has a very high "coefficient of restitution") up snuggly to his forehead with a large caliber wadcutter made of hardened steel to knock him down. Otherwise it is not likely.
No handgun round has enough momentum to knock down a human being. Unless they're off balance. Hell, a 12ga slug doesn't have enough momentum even if all the "energy is expended into the target."