Poking around on Paul's site (because what else am I going to do on a Thursday night?
), I noticed that the XM8 entry in Best Assault Rifles That Never Were has the Carbine, Compact Carbine, and Designated Marksman variants, but not the Automatic Rifle. That variant had a heavy 20" barrel and could use a 100-round drum magazine for sustained fire. It also had a bipod (per HK Systems, the 12.5" and 20" barrels had bayonet lugs and only the 20" barrels had a bipod interface). It appears to be a different 20" barrel than the DMAR, based on the venting at the end of the barrel.
HK Systems also listed a 10 round magazine that isn't in the BARTNW entry. That seems a bit small to me, but maybe there'd be a use for it that's not immediately obvious.