Ho-kay, I put together an SBR upper off of a 1-in-9" Bushy barrel that began life as a 14.5", cut & recrowned to 12.5" (my “learning upper,” using a barrel I got for free; in case I ganked it up, I’m not out any real $$$). FSB ground down to fit a Troy 10" rail onto it. Everything is mechanically functional, so…
Slapped on front & rear Magpul MBUS, took it to zero @ 50yds. Azimuth, I’m fine, took some leftward adjustment; no drama. Elevation, I’m still about 4.5" low, and the post is BURIED; no more room for movement. If it matters, ammo used was Centurion 55gr ball (M193 clone ammo).
Before I go and take a file to the MBUS front sight post, only to find out later that there was a better solution, my questions are:
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Which, if any, folding BUIS fronts have a greater range of adjustment (downward on the post, upward on the strike of the round) than the MBUS?
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If SOL on the front, am I better off getting a KAC rear BUIS or similar that has a height adjustment and kick it upward a notch or three?
I’m not gonna put a solid front post on this thing, as I take the BU part of BUIS seriously. Those who use folding BUISs on their SBRs, what do you use?
in the spirit of not addressing the original question, are you sure your rail is straight, and isn’t canted upward at the muzzle end? i’ve no experience with troy, so i dont know how likely it is something got crooked, but i figured i’d throw that up.
Nope, that’s a good point, and something I should have thought of, having seen that very question asked before on this very site. Teachable moment…
Not that I can tell just looking at it, but I’ll get it checked out by somebody with a bigger brain and better diagnostic savvy and go from there.
which model troy/how does it fasten? it’s (obviously) a free-floating model, right?
its a possibility- i’ve had a few rails of different make/model that went on crooked. sometimes by my own hand and not paying attention, and sometimes by the rail’s own fault. the clamp-on type rails seem to be a bit easier to install funky.
since you’re not zeroed anyway, i’d suggest pulling the sights off and setting the upper upside down on a nice known-flat surface. check to make sure it’s on the same plain, and that it’s linear.
if it’s totally straight/plumb, the sights themselves are probably the next most likely culprit. i’ve seen a couple mbus that had to go back… being polymer, i think it’s inevitable that they’ll pop a couple out a little jacked up.
10" MRF-MX, and I just got done yanking the sights off and slapping the upper against my 4’ level while you posted the above. The “tinkering” gene surged to the forefront…
Sure enough, there’s a gap of +/- 1/8" in the center of where the back of the upper receiver and the front of the rail rest on the straightedge. This just stopped being an Optics & Mounts thread! 
I had the fixed FSB on this thing ground down to accept a free-float rail, and appears as though it could be the culprit. Not much room, presuming that it isn’t completely in the way, so if I can’t get it safely ground down further, I need to invest in a lo-pro gas block to ensure there’s room for this thing to get kicked down into true.