Just weighing in on the MBUS for some... I bought a FS, and as soon as I mounted it, it wouldn't flip up from the left or sit flush. I returned it, got another one which laid flatter, but not flush, and still would not pop using the left tab.
So for my experience Magpul has build specs that have some play in them and my railed hand guard (Troy VTAC) picatinny is too wide/out-of-spec. I've decided that my rail is the bigger issue, since the rear sight slides right on and off of the upper, but is too snug on the hand guard.
As for the windage, I found that with the detachable handle I had, mounting and re-mounting with different ways of tightening the screws would change how much windage I needed to zero the rifle.
So, if yours slides around, maybe it's torquing in the picatinny when you tighten it down and throwing your windage off?
was it a gen 1 or 2 sight? i have the gen 2.
