M5 quad is fearsome, now AA HT, so flak halftruck is next?
Why 251/17 should attack on move? It does not use MGs, but Flak gun.
Yet, 251/17 is deadly while Quad and AA are not that deadly.
Show me a game where AA can achieve over 50kills. You won't find it, but you will find plenty games with such amazing 251/17 usage.
It can kill whole squad in 1 burst. That's enough reason to make it stationary.