I read in another forum that modifying an AR-10 to take an AR-15 DIAS (or modifying the DIAS perhaps, don't recall) would be considered manufacturing a post-'86 MG because the DIAS wasn't designed for the AR-10.  What if the DIAS goes in an AR-15 lower per-specs, but is mated to an upper built/assembled to fire 6.8 SPC or 6.5 Grendel, etc.?  Is it just the lower's physical specs that count, or some combination of upper/lower, or some nomenclature BS, or something else according to the local BATFE expertise on tap?  

I would say that since the RDIAS is the reg. part,
you shouldn't have a problem.

Notice I said 'shouldn't'.

Might be a case for contacting ATF-I would think it's only "manufacturing a post '86" if you modify the RDIAS as it's the part considered a machine gun,and can be moved from "host rifle to host rifle" , but don't take that as advice, people have been arrested for much simpler things firearm related.  One thing to keep in mind before modifying your rifle to make it fit, though, is whether FA carriers are available for the AR10. If M16 carriers will fit it's not an issue, but if they won't I have never seen a FA carrier to fit the AR10.

I should clarify, I only mentioned the AR10 as a potentially comparable reference point since it was mentioned as illegal to put an AR-15 DIAS in one.  My interest (someday) would be in an AR-15 spec lower receiver mated to what I assume would be an inherently mechanically compatible AR-15 upper which was barreled and bolted to fire 6.5 Grendel, say, a future Patriot Ordinance Factory development if I'm lucky.  Basically, if everything fits together as normal and functions 'cause the parts besides the bolt and barrel and mag are an iteration of the AR-15, but just in a different cartridge, is that suddenly a New MG to the ATF?  hanks