See 26 United States Code § 5845(b) :
(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.
So according to federal law, a sear is a machine gun in and of itself. "Machine gun sear" is a description; "machine gun" is the actual legal classification for a sear.