Here's the legal definition of MG:
"(b) Machinegun
The term “machinegun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. 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."
If ATF can make your seized gun shoot more than one shot when the trigger is pulled, by using or modifying any parts you have in your possession, assembled or not, you have a MG.