Example of Knockdown power:
Professional football offensive lineman picks up a 25lb medicine ball and throws it at a 100lb womans chest as hard as he can. She catches it but gets knocked down.
Example of Stopping power:
Direct fire from a 155mm howitzer. Even a dumby round to the chest will stop any human being immediately.
Example of no appreciable knockdown or stopping power:
BB fired at 120fps.
Example of both knockdown and stopping power:
18lb cannon ball fired with a slight undercharge (so it doesn't overpenetrate). If it hits a person in the chest they are dead and also knocked back.
That establishes that both these things exists, and that their is a point where they have no appreciable affect on humans, and a point where each affect is completely guaranteed. That small arms fall somewhere on the spectrum between these extremes is a given. Determining where is pretty easy for knockdown power, as small arms are clearly barely better than a BB in that category. Stopping power on the other hand kicks in much faster, so basically all small arms have some measure of stopping power. Determining how much is virtually impossible though, so all you can do is rate them relative to each other based on certain factors, but even that is extremely hard to do unless one round is clearly better in all ways (larger caliber, higher velocity, greater weight, all at once).
Also shot placement plays NO PART in determining stopping power. That is a different subject entirely. If someone brings that up in a discussion of stopping power that means they have a basic misunderstanding of what is being discussed.
