Moving a person sized body with a bullet just isn't possible (Mythbusters did a pretty good job covering it in this episode).
While it is a good episode of Mythbusters, it is largely irrelevant as a counter argument to Takedown. Takedown doesn't throw the guy back several feet, it drops him sprawled in his square, which is exactly what Buster looks like he is doing when he gets shot.
That's because he's on a hook and can't stand up on his own legs. He moves a millimetre and falls due to being unable to support himself. If he had leg muscles in the test, rather then a hook, he wouldn't have moved at all for anything short of a large gauge shotgun or a .50BMG (and even then it would have been a very small distance).
That's due to shock and trauma
And how is shock and trauma not a Fortitude saving throw?
Fortitude saves are defined (in Spycraft 2.0) as being "used when he resists physical attack or influence, such as a contagion, critical injury, or the concussive force of some powerful attacks."
By that logic, all firearms (and probably all forms of physical damage - grenades especially) should have takedown. Which they don't. The difference from a kinetic standpoint between a 9mm and a .45 is very minor, just due to the mass of the target. I sure as hell don't want to have to make a "trauma fort save" every time I am attacked by the GM, and I don't want players making them every time they are attacked.
Not only does it slow down combat, it also makes little sense in the framework of the game - given the definition of vitality*, Takedown should only ever come into play on a critical hit or when the target is out of vitality. Just when they actually become injured (and can then make their "trauma save"). Which is something tfwfh already suggested, and something I still like.
These are a mixture of endurance, luck, and the will to fight, measuring a character’s ability to avoid injury
. Losing vitality does not represent actual physical damage
but rather combat fatigue, as it gradually becomes more difficult for the character to avoid being hurt.
Emphasis is mine.