Well, BATFE received this today, as part of their Bumpstock Ban regulation comment period:http://www.feinsteinproject.org/loes/effortbumpfire/TrumpStockMk1FingerShelfLeftSide.jpg
If my MOC is declared a machinegun by the government, there could be a slight issue in our joint hobby:
(From the Federal definition of a machinegun)
The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled
if such parts are in the possession or under the control of a person.
I think there are already illegal LEGO assemblies, as Technic bricks have the perfect diameter hole to make air rifle suppressors, so possession of Technic bricks and a .177 air rifle may already be a felony.