Perhaps you are not reading the right section:
TITLE 18 > PART I PART ICRIMES
(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.
Once again, with emphasis, whether we're talking about taxing firearms, or defining the criminal use thereof, an "antique firearm" IS NOT a firearm.
BTW, if you don't know why I'd be quoting tax law regarding firearms, you really need to quit pontificating and familiarize yourself with the subject. BATFE (you do know who they are, don't you?) is essentially a tax collection agency.