EB,
Here's the definition of a firearm from 18 US Code: no mention of barrels.
Regards, Tim
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TITLE 18 > PART I > CHAPTER 44 > § 921Prev | Next § 921. Definitions
How Current is This? (a) As used in this chapter—
(1) The term “person” and the term “whoever” include any individual, corporation, company, association, firm, partnership, society, or joint stock company.
(2) The term “interstate or foreign commerce” includes commerce between any place in a State and any place outside of that State, or within any possession of the United States (not including the Canal Zone) or the District of Columbia, but such term does not include commerce between places within the same State but through any place outside of that State. The term “State” includes the District of Columbia, the Commonwealth of Puerto Rico, and the possessions of the United States (not including the Canal Zone).
(3) The term “firearm” means
(A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive;
(B) the frame or receiver of any such weapon;
(C) any firearm muffler or firearm silencer; or
(D) any destructive device. Such term does not include an antique firearm.