OK, unusual question here.  I have an aimpoint that I currently mount on an MP5 using a combination  of a low mount (BT10266) and a QD ring (BT21530).  The combination allows me to co witness, and I have the aimpoint adjusted such that the red dot sits right on top of the front post.

I wish to mount the same Aimpoint/QD ring onto my AR flat top (Also using BUIS that is same height as A2 rear sight) and need to find a mount/riser that will allign the aimpoint to cowitness without the need to adjust the aimpoint.

Does anyone have this compination who can offer their experience with such a mount?

The next time I go to a gun show, I will take along both firearms and see if a table holdee will allow me to ry out various mounts, but I thought I'd ask here first.

Have you checked out our optics forum? It’s dedicated strictly to this type of question. In short what your looking for is a fixed rear BUIS like the one LMT offers. As far as Aimpoint mounts go, the standard Aimpoint QRP mount without spacer will give you a absolute co-witness.