; (B) the term major component , with respect to a firearm— (i) means the slide or cylinder or the frame or receiver of the firearm; and (ii) in the case of a rifle or shotgun, includes the barrel of the firearm; and (C) the term undetectable firearm means a firearm, as defined in section 921(a)(3)(A), of which no major component is wholly made of detectable material; ; (4) in paragraph (3)— (A) in the first sentence, by inserting , including a prototype, after of a firearm ; and (B) by striking the second sentence; and (5) in paragraph (5), by striking shall not apply to any firearm which and all that follows and inserting the following: “shall not apply to— (A) any firearm received by, in the possession of, or under the control of the United States; or (B) the manufacture, importation, possession, transfer, receipt, shipment, or delivery of a firearm by a licensed manufacturer or licensed importer pursuant to a contract with the United States. . 517. Prohibition on possession of certain firearm accessories Chapter 44 of title 18, United States Code, is amended— (1) in section 922, as amended by section 516 of this Act, by adding at the end the following: (dd) (1) Except as provided in paragraph (2), on and after the date that is 90 days after the date of enactment of this subsection, it shall be unlawful for any person to import, sell, manufacture, transfer, or possess, in or affecting interstate or foreign commerce, a trigger crank, a bump-fire device, or any part, combination of parts, component, device, attachment, or accessory that is designed or functions to materially accelerate the rate of fire of a semiautomatic rifle but not convert the semiautomatic rifle into a machinegun. (2) This subsection does not apply with respect to the importation for, manufacture for, sale to, transfer to, or possession by or under the authority of, the United States or any department or agency thereof or a State, or a department, agency, or political subdivision thereof.