Certainly not an expert, but I did stay in my own house last night.  ;^)  
I have a Nightforce 5.5x22x56 on my AR10 on Leupold steel high rings and it just clears the Pic rail of the KAC RIS handguard.  So close that I cannot get the rubber NF scope cover on the objective lens without trimming off some of the rib on the cover.  I do know the Larue Tactical LT104 will work with this combo, but haven't purchased one just yet due to an upcoming address change.
If I had to do it over, I would probably buy the 50mm objective to allow more choices of mounts, but I do like the bright image the 56 affords.  I'm not really sure if there is all that much difference between the 50 and 56 in light gathering capability, as the NF optics are superb in clarity.  Perhaps someone that has done side by side can comment further on this in regard to the scopes.
I do know I would purchase the NF brand again, as I have not looked through anything that I can discern to be of better quality to this point.
BTW if you need/want to have a 10 or 20 degree canted mount for long range shooting (not required on the NF with 100 moa vertical), definitely go with the 50mm, as the 56 would probably be against the rail with a lens shade in place.

good points, thanks!