I have searched the optics forum and read most of the FAQs looking for an answer to what is probably a very simple question, but I am still confused.

Here's what I have: a mixmaster with a CMMG 14.7" A3 flattop upper. It has a tall "F" front sight base.

Here's what I want to do: Mount an EoTech 552 on it and have a co-witness capability with an ARMS 40-L SP or Matech rear BUIS.

Here are my questions:

1. What mounts are available for the Eotech to put it at the proper height to do this? I am thinking a        
   1/3 lower co-witness is best.

2. What mount is best? Best to me being most durable and good return to zero.

3. What mount does the military use and do the strive for a co-witness set-up?

4. Is either BUIS (ARMS or Matech) more suitable for this purpose?

Thanks in advance for your patience and help. Andy

I have a colt 6920 with a F marked FSB and have the eotech 552 and matech BUIS mounted directly on the a3 rail.  It is an absolute co-witness, but there is not a lot of room to hit the buttons on the eotech.  I've never used the arms BUIS (heard good things about it though) but I really like the Matech.  My groups tightened up better than using the carry handle sight.  Hope this helped answer your questions.

Thanks for the reply RPJ. I have a 6920 as well, but I decided to put this set up on my mixmaster, because it actually shoots slighty better groups (both w/ A3 carry handle sights). Anyhow, can the Eotech be mounted directly to the rail? I thought I would need an adapter. If I don't that would be sweet! Thanks, Andy

The EOTech, when mounted directly to the A3 rail (using built in mount), will co-witness with standard sights.

If you want lower thrid irons, you need to use a riser. Depending on your price/weight, etc. you can choose from YHM, RRA, LaRue, ARMS. I agree with C4iGrant that LaRue makes the best one out there. I do not have one on my rifle (direct mount), but have seen and played with them in action. The ARMS is also nice, but personal preference says LaRue.

Quoted: The EOTech, when mounted directly to the A3 rail (using built in mount), will co-witness with standard sights. If you want lower thrid irons, you need to use a riser. Depending on your price/weight, etc. you can choose from YHM, RRA, LaRue, ARMS. I agree with C4iGrant that LaRue makes the best one out there. I do not have one on my rifle (direct mount), but have seen and played with them in action. The ARMS is also nice, but personal preference says LaRue.

Exactly. You explained it perfectly.

I went with the Eotech 552 and the Matech BUIS, no riser. I am thrilled with the set-up. Thanks everyone!