On stocks, there's a kind of paradox I can't explain.
I took some dial calipers and measured from the comb of the stock to the middle of the rear sight aperture on an AR15A2.
Then I took the dial calipers and measured from the comb of the stock to the middle of the Aimpoint on the XCR.
Both the XCR and the AR15A2 have fixed stocks, so we're comparing apples to oranges. The XCR uses the M4 fixed stock adapter.
What I noticed is that the AR15A2 measurement is just a touch more than a quarter inch lower than the XCR measurement. Add to that the fact that my Aimpoint mount is the LaRue shorty CCO (not the full 1.410" like you'd run on a flat-top AR) and it's even more interesting.
The paradox comes in that the XCR actually comes up more 'centered' for me. When I pivot the XCR up, the dot is centered in the Aimpoint just right for snap shots. Why? I have no idea. As well, I thought that going from the XCR factory tubular folding stock to the fixed AR stock would raise my cheek somewhat - and it did appear to do so! But it didn't change anything as far as how the Aimpoint comes up to my eye.