The Armalite-pattern uppers use a 1-7/16"x18 pitch thread which require a corresponding barrel nut/handguard setup to mount up correctly. The DPMS/LR-308 pattern uppers use a 1-7/16"x16 pitch thread pattern. Thus, they are NOT compatible/interchangeable in terms of handguards/rails. Furthermore, the rail height on the Armalite handguards won't match up to any of the existing LR-308 receiver heights which will just further add insult to injury with trying to put a proverbial square peg in a round hole.
There's hardly a shortage of quad-rail, DPMS-pattern (in varying corresponding heights) handguards/rails on the market, so pick one that best "looks" the part of the AR-10NM unit to your eyes and move on, but I wouldn't go attempting to shoehorn an Armalite handguard onto your DPMS upper.