That sounds
exactly like BRP Corporation's XMG beltfed MG34 upper.
OK, so you take a title one firearm, (ROGAR 15), which is what's being sold, you add a device, in this case a M-16 lower, to make the title one gun function as a full auto weapon. You've just created a post sample machine gun from the ROGAR 15. Unfortunately, the ROBAR 15 is a gun in and of itself.
While I don't agree with this ruling, that certainly seems to be BATF's new logic with the BRP upper which is itself registered as a title one weapon...
I hope I'm wrong.
Please read this thread.
http://www.uzitalk.com/forums/showthread.php?t=41910