18 U.S.C. § 922 : US Code - Section 922: Unlawful acts:
(p)(1) It shall be unlawful for any person to manufacture,
import, sell, ship, deliver, possess, transfer, or receive any
firearm -
(A) that, after removal of grips, stocks, and magazines, is not
as detectable as the Security Exemplar, by walk-through metal
detectors calibrated and operated to detect the Security
Exemplar; or
(B) any major component of which, when subjected to inspection
by the types of x-ray machines commonly used at airports, does
not generate an image that accurately depicts the shape of the
component. Barium sulfate or other compounds may be used in the
fabrication of the component.
(2) For purposes of this subsection -
(A) the term "firearm" does not include the frame or receiver
of any such weapon;
(B) the term "major component" means, with respect to a
firearm, the barrel, the slide or cylinder, or the frame or
receiver of the firearm; and
(C) the term "Security Exemplar" means an object, to be
fabricated at the direction of the Attorney General, that is -
(i) constructed of, during the 12-month period beginning on
the date of the enactment of this subsection, 3.7 ounces of
material type 17-4 PH stainless steel in a shape resembling a
handgun; and
(ii) suitable for testing and calibrating metal detectors:
Provided, however, That at the close of such 12-month period, and
at appropriate times thereafter the Attorney General shall
promulgate regulations to permit the manufacture, importation,
sale, shipment, delivery, possession, transfer, or receipt of
firearms previously prohibited under this subparagraph that are
as detectable as a "Security Exemplar" which contains 3.7 ounces
of material type 17-4 PH stainless steel, in a shape resembling a
handgun, or such lesser amount as is detectable in view of
advances in state-of-the-art developments in weapons detection
technology......