i have an aimpoint on my AR, but also am buying a scope for clearer long range work, are there any mounts with see through undersides for a flat top / buis set up?  I noticed in shotgun news that the tri-rail for the flatt op has see through interior.

Anything that puts your scope up so high that you can see the irons underneath will be as tall as if it were mounted on a carry handle.  Bad thing IMHO.

I prefer using a QD mount for a magnified optic and fold-down BUIS.

If you just gotta go that route though, YHM makes an odd looking unit that mounts to the flat-top, raises your scope over your sights, and allows for additional mounting on the sides.  As with everything else YHM offers, there's a half-dozen sling mounting points as well

Yankee Hill

what happens to the front sight (fixed) with a low mounted scope?

Above 3x or so, it disappears... May or may not see a slight shadow in the lower 1/4 of the scope.  Above 4x, you won't see anything at all.

gonna post new topic re: mount

I can tell you from experience that at 5x it disappears completely.