Using your BUIS and Aimpoint's T2 optic
I have tested this at 25 and 50 yards. At 50 yards, I get a 2" shift. At 25 yards, I get a 1" shift.
When shooting with irons, looking THROUGH the T2, POI is shifted up and to the left. When shooting with the T2 optic removed, it is shifted down and to the right.
It is a consistent 4 MOA shift, in my testing. I tested with different ammo, and on different days. The results were always the same.
This is something to be aware of. Zero and shoot accordingly.
http://i62.tinypic.com/288oxte.jpg