I own a Bushmaster M4A3. I bought a AimPoint CompM3 sight. I have it mounted on the flat top by using a Arms #22M68 Mount and a back up Arms #40L-SP Iron sight. My question is they recommend you use there full spacer which I did, but I noticed when I would use the iron sights it would sight threw the bottom portion of the AimPoint (It was to high up). From there I figuered there half spacer (the one with the half circle holes) should line my sight up better. Which it did. So now if I have the red dot on and look threw it right in the middle, if I pop up the iron sights it all co witnesses perfectly. Is it because I'm using there #40L-SP Iron sight, which they are not taking into acount when they wrote the following on there web page....

Designed exclusively for the Aimpoint® Comp. ML 1, ML 2 and ML 3 sights.
Low std. height allows perfect Red Dot co-witnessing on M16/M4 with A.R.M.S.® #38™ Sleeves and S.I.R.® without a spacer. A full spacer is required if mounting directly on the flat top rail.

This looks to be the same set-up I have...
http://www.armsmounts.com/catalog.php?action=124&item_id=21
But they are using that full spacer. Even in that picture the rear sight seem's to be sitting lower then the middle of the AimPoint. Let alone I would like to get the AimPoint 3x Magnifier. But the spacer you should use is the#22M68 spacer which is a 1/2 inch rise, like there full spacer (which will throw everything off again.

I even notice when I do quick draws I'm on target faster with the half spacer then the full one. Can someone please tell me what I'm doing wrong and explain this all to me and tell me what I should do.

Thanks in advance for any help.

Thanks for clearing that up for me. I think I will try it both ways till I figure what works best for me. I can a figured if I got the 3X magnifier I would have to get use to a 1/2 inch space.