Actually, you can reload and still get the no-miss-whole-clip challenges. I've gotten "Perfectionist" 3 or 4 times now, and I'm 200% certain that one of the times I had shot two, reloaded, shot two, reloaded, and it still unlocked. So, it actually keeps track of the max size of the magazine of your primary weapon... and tracks if you hit that many shots in a row with it.
Quid quid Latine dictum sit, altum videtur.
Killing with a vehicle mounted gun is nothing, doing it while airborne is impressive, but upside down while airborne? F***, THAT's an achievement!