I want to buy a comp m4s for my colt 6920 and would prefer a lower 1/3rd co-witness. Is the factory aimpoint mount (QRP with spacer) lower 1/3rd or is it absolute co-witness? I have had ADM, Bobro, and Larue mounts on scopes and really prefer the bobro, but I do not think their mount is lower 1/3rd even with the factory aimpoint spacer. Will the ADM SOCOM lower 1/3rd riser/spacer work with the bobro mount? I called ADM and they could not answer, neither could Bobro. Thanks.

With the spacer the mount is absolute.
LaRue makes a very nice lower 1/3 mount if you want something higher.

I'm pretty sure the stock spacer actually puts the sight in between absolute and lower 1/3.  I know the M4 on my work rifle with the stock QRP mount/spacer definitely sits higher than absolute.

It is 1.535" above the rail with the spacer. Iron sights are 1.405" above the rail so it isn't perfectly aligned with the irons but anything around 1.5" above the rail is called "absolute co-witness". For reference, LaRue's "lower 1/3" is 1.725" above the rail.

The ADM spacer looks like it works with an ACOG style mount to me.

Looks like this would work for you (says it puts the height at 1.65" above the rail):

http://americanshootingjournal.com/mounting-solutions-plus-lower-13-co-witness-spacer-for-aimpoint-pro/

Thanks!

Don't rule out the Scalarworks Comp M4/M4s mount if looking for lower-1/3. I have one for the T1 and it's awesome, though I'm using the LRP for my M4s.

https://scalarworks.com/shop/optic-mounts/ldm-compm4/

Qrp always seemed between absolute and lower 1/3 to me. I got a larue lt659 lower 1/3 mount and it does what I need it to. Have several larue mounts and have never had a reason not to give them my money.