, it is very unlikely to physically slam a person back and down right now. If it won't do that, and if I can use something else that will, well ...., which would you pick?
Lucky Guy, I think you've been watching too many movies, nothing one can carry will actually "slam a person back and down", if you want that effect you'll have to hit them with your car. I do agree that within reason, bigger is better, and faster is also better. A 200 grain .45 is a better stopper than a 200 grain .38 and a 125 grain at 1500 fps is better than a 125 grain at 900 fps.
I also agree with Duane that shot placement is more important than the size, weight or velocity of the slug. The problem with that is that all your shooting skill goes out the window when a panic reaction sets in. Modern day cops are very well trained, accustomed to confrontation, and yet, in an actual gunfight, they miss three times out of four. Shooting an armed opponent between the eyes is also something that seldom happens outside the movies.
Yet, a movie gave the best information on gunfighting I think I've ever heard. In the movie "Unforgiven", Gene Hackman gave a dissertation on gunfighting like this. "It ain't easy to kill a man, especially if he's shooting back, that just plain rattles most folks." "Being good with a gun, fast and accurate, that don't do no harm, but it really don't amount to much against being cool headed, the man who can hold his nerve under fire, like as not he'll kill you." So Duane, if you're that "cool headed" you'll do just fine with a .22, but the rest of us "most folks" would do better with something more powerful.