After reading NewArguy's post about the TA01-NSN/CompM combo, I just tried the EoTech 551 in front of the TA01-NSN on a RRA flattop with a Bushmaster 14.5"/Phantom barrel, with interesting results. Hopefully others with different parts laying around can try it and see if what results you get. This was a temporary setup, with items just finger-tightened.
First, you would need a longer rail like an extended #38, a SIR, etc. With the NSN as far back as it could go (no BUIS), the little Eotech ran out of room on the upper- The single crossbar that is designed to go through the flattop rails was actually just in front of the rail, over the delta ring. But I could at least tighten both ends of the EO mount enough to make it secure for this test. Obviously you wouldn't want to actually shoot in this condition.
The rubber EO buttons were right up against the front of the NSN, but I was able to turn the unit on (Glad I don't have big fingers!) The EO reticle showed up clearly, bright, and actually looked like part of the NSN. The view was awesome!
My biggest problem:
The Eotect reticle wasn't co-witnessing, it is down and to the left of the NSN's crosshairs. Maybe because of the half-ass way the EO was mounted?
(I am also having problems with this same unit co-witnessing on just a basic front sight and an Arms-40A; I tried two different uppers with the 40A. Could the EoTech be off? I turned the windage/elevation over 5 full turns, and the 65moa ring doesn't seem like its moving. Can any EoTech pros help with this one?)
I'll post some photos of this tommorrow.