Harrekin is 100% right and spot on.
15 U.S.C. § 5001 : US Code - Section 5001: Penalties for entering into commerce of imitation firearms
(a) Acts prohibited It shall be unlawful for any person to manufacture, enter into commerce, ship, transport, or receive any toy, look-alike, or imitation firearm unless such firearm contains, or has affixed to it, a marking approved by the Secretary of Commerce, as provided in subsection (b) of this section. (b) Distinctive marking or device; exception; waiver; adjustments and changes (1) Except as provided in paragraph (2) or (3), each toy, look- alike, or imitation firearm shall have as an integral part, permanently affixed, a blaze orange plug inserted in the barrel of such toy, look-alike, or imitation firearm. Such plug shall be recessed no more than 6 millimeters from the muzzle end of the barrel of such firearm.
Emphasis added.