Originally Posted by bret van
With the UTG mnt910 rear aperture you get a different sight pic. when I look above my rear sight the dot moves above the front iron about 1/4'' it the is almost centered in the tube and my rear sight is a blur.
What you have is commonly called a "lower 1/3 co-witness". If your red dot and your open sights are both zeroed at the same range and you are able to achieve a proper sight picture with your open sights (front post centered within the peep aperture) with the RDS in place, then the red dot will always appear at the top of the front post. But you will be looking at the open sights through the bottom of the field of view of the optic and the red dot will not be centered within that field of view.
If the sights and optic are aligned such that the open sights are lined up when the red dot is centered in the optic, it is called an "exact co-witness".