I have a Burris Fast fire III red dot with Flat Top adapter and a 1/2" Yankee Hill Machine riser, sitting on my BR99 shotgun.
I've had to raise it like that because the shotgun has a comb that is almost straight with the factory rail.
The trouble is, I am hitting 6" low at 25 yards and I have run out of UP adjustment on the Burris. The range of elevation adjustment on the Burris is 115 MOA.
It seems to me I need an angled Picatinny riser, one that is slightly raised at the back and slopes downward to the front.
Here are pics of my current setup:
