H.R. 4221 — Undetectable Firearms Modernization Act

This section revises the prohibition on undetectable firearms (18 U.S.C. §922(p)) by (1) redefining a firearm as undetectable if, after removing all parts other than major components, it does not contain detectable material (previously, if not as detectable as the Security Exemplar by walk-through metal detectors) and if it would not generate an image by airport detection devices (previously, x-ray machines); (2) defining major component as the slide or cylinder, frame or receiver, or—for rifles or shotguns—the barrel, and detectable material as any that produces a magnetic field equivalent in strength to 3.7 ounces of 17-4 PH stainless steel (replacing prior definitions); (3) extending the prohibition to prototypes of a firearm and striking a prior exception for prototypes reduced to non-firearm production; and (4) limiting exemptions to firearms possessed by the United States or manufactured, imported, possessed, transferred, received, shipped, or delivered by licensed manufacturers or importers pursuant to an existing U.S. contract (replacing prior exemptions). (Thus, the changes modernize detectability standards to current airport screening technology, broaden coverage to emerging manufacturing like 3D printing, and narrow non-government exemptions.)