Thanks for the reply Jack. I appreciate the info.
What I don't understand is why I should use the full height spacer when my rifle has flip-up front and rear sights.
Please understand that I don't want to come off as argumentative about this, I just want to know if I use the half-height spacer, would that place the irons in the middle of the Aimpoint's glass?
Almost every AR I've seen with a full height spacer also has a fixed front sight. Alternatively, I have seen an AR with a half-height spacer but, unfortunately, the pic does not show the front sight. If I had a fixed front sight, I would understand why I should use a full height spacer.
BTW, FWIW, I should have mentioned that I would want to mount the Aimpoint on my FF RAS so I wouldn't need to use the cantilever mount.
Again, thank you for the response. I was beginning to think I was never going to get any replies [:(]