I have had decent luck w/ my carry handle returning to zero when i tried it out. the acog when remounted would not return as close to zero however, but that might be because i am able to achieve a finer aiming point.
we were not issued backup sights w/ the acog, but i put my own on my rifle because its just plain stupid to put all your eggs in one basket when your life might be on the line.
Interesting! Thanks for the reply Nick!