I am working on a custom weapon, I want to use AR15 BUISs for sights, therefor I need to know exactly how much lower the railed gas block is compared to the A3 flat top upper. (so that I can build my front/rear rails at the same difference.) You can find out by puting a straight edge on the flat tops rail, then seeing how big the gap is between said straight edge and front gas  block.

My other opyion is: who make a non-folding front BUIS that fits on the railed handguard at the same height as the uppers deck?
Thanks.

they make some that are for going on the end of free float tubes you could use those, and those would be the same height, as FF tubes are the same height as the upper height, dont know who makes em though but they are out there.