H.R. 4221: Undetectable Firearms Modernization Act
Sponsor
Madeleine Dean
Democrat · PA-4
Bill Progress
Latest Action · Jun 27, 2025
Referred to the House Committee on the Judiciary.
Why it matters
Introduced on 2025-06-27, the bill would tighten federal rules for firearms made with modern materials and designs that can evade airport-style screening and metal detectors.
The exemptions are narrow and government-focused. Firearms received by, possessed by, or under the control of the United States are exempt. So are firearms manufactured, imported, possessed, transferred, received, shipped, or delivered by a licensed manufacturer or licensed importer under an existing contract with the United States. The bill does not include any new funding, grant program, civil fine schedule, or implementation deadline.
What does H.R. 4221 do?
Sets a 3.7-ounce detection benchmark
The bill defines 'detectable material' as material that produces a magnetic field equivalent in strength to the field produced by 3.7 ounces of 17–4 PH stainless steel, giving federal law a specific measurable standard.
Targets airport-screening evasion
It bans firearms that, when inspected by the types of detection devices commonly used at airports for security screening, would not generate an image that accurately depicts the shape of the component.
Defines major parts for rifles and shotguns
The bill says a 'major component' means the slide or cylinder, or the frame or receiver, and for a rifle or shotgun it also includes the barrel, clarifying exactly which parts matter under 18 U.S.C. § 922(p).
Covers any firearm, including a prototype
The prohibition is expanded to apply to any firearm, including a prototype, which means experimental or not-yet-commercial designs are explicitly covered.
Removes old Treasury testing exception
The bill strikes the second sentence of paragraph (3) of 18 U.S.C. § 922(p), removing language that had previously provided exceptions for firearms tested by the Secretary of the Treasury.
Keeps exemptions tied to U.S. contracts
It preserves exemptions for firearms received by, possessed by, or under the control of the United States, and for actions by a licensed manufacturer or licensed importer under an existing contract with the United States.
Who benefits from H.R. 4221?
Airport and building security personnel
They would get a clearer federal rule aimed at firearms that evade the kinds of detection devices commonly used at airports for security screening, including a specific 3.7-ounce 17–4 PH stainless steel detectability benchmark.
Law enforcement and prosecutors
They benefit from more precise definitions in 18 U.S.C. § 922(p), including what counts as a 'major component' and the explicit inclusion of any firearm, including a prototype.
The United States government and federal contractors
The bill keeps exemptions for firearms under the control of the United States and for licensed manufacturers or licensed importers operating under an existing contract with the United States.
Public users of secured spaces
People in airports and other screened locations could benefit if the updated rules make it harder to bring in firearms that do not show up clearly on metal detectors or X-ray screening systems.
Who is affected by H.R. 4221?
Firearm designers using low-metal or nonmetal parts
They would face stricter federal limits if their firearms lack detectable material outside major components or fail airport-style imaging tests under amended 18 U.S.C. § 922(p).
Prototype and experimental firearm makers
They are directly affected because the bill says the prohibition applies to any firearm, including a prototype, eliminating ambiguity about test models.
Licensed manufacturers and licensed importers
They would need to ensure products comply with the revised standards unless they are acting under an existing contract with the United States, which remains exempt.
Owners or possessors of covered firearms
Anyone dealing with firearms that do not meet the new detectability and imaging rules under 18 U.S.C. § 922(p) could be affected by the broader federal prohibition, though HR4221 itself does not add a new penalty amount in the text provided.
H.R. 4221 Common Questions
How much metal would a gun need to be detectable under HR 4221?
Under the Undetectable Firearms Modernization Act, "detectable material" must produce a magnetic field equal to 3.7 ounces of 17–4 PH stainless steel (Section 2(2)(B)).
Can prototype guns be banned under the Undetectable Firearms Modernization Act?
Yes. The bill expressly applies the undetectable firearms ban to any firearm, including a prototype, according to HR 4221 Section 2(3)(A).
What gun parts count as major components under HR 4221?
Under the Undetectable Firearms Modernization Act, major components are the slide or cylinder, the frame or receiver, and for rifles or shotguns, the barrel (Section 2(2)(A)).
Does HR 4221 cover rifles and shotguns as well as handguns?
Yes. According to HR 4221 Section 2(2)(A), rifles and shotguns are specifically covered because their barrel is defined as a major component.
Can a firearm be illegal if airport screening devices cannot show its shape?
Yes. Under the Undetectable Firearms Modernization Act (Section 2(1)(B)), a firearm is prohibited if airport-style screening devices would not generate an image depicting its shape.
Does HR 4221 apply to more than just x-ray machines?
Yes. HR 4221 broadens the test from "x-ray machines" to detection devices commonly used at airports for security screening, under Section 2(1)(B).
Can the U.S. government still possess firearms that would otherwise be undetectable under HR 4221?
Yes. Firearms received by, possessed by, or under the control of the United States are exempt under the Undetectable Firearms Modernization Act (Section 2(4)).
Can licensed gun makers manufacture covered firearms for the government under existing contracts?
Yes. Under HR 4221 Section 2(4), licensed manufacturers and importers may manufacture, import, transfer, ship, or deliver them under an existing U.S. contract.
Does HR 4221 remove the old Treasury testing exception for undetectable firearms?
Yes. The bill strikes the second sentence of 18 U.S.C. 922(p)(3), removing prior exception language, according to HR 4221 Section 2(3)(B).
Does HR 4221 still use the old Security Exemplar standard for metal detectors?
No. Under the Undetectable Firearms Modernization Act (Section 2(1)(A)), the bill removes the Security Exemplar calibration and replaces it with a detectable-material test.
Based on H.R. 4221 bill text
HR4221 Legislative Journey
House: Committee Action
Jun 27, 2025
Referred to the House Committee on the Judiciary.
About the Sponsor
Madeleine Dean
Democrat, Pennsylvania's 4th congressional district · 7 years in Congress
Committees: Foreign Affairs, Appropriations
View full profile →
Committee Sponsors
Judiciary Committee
0 of 42 committee members cosponsored
No committee members have cosponsored this bill
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H.R. 4221 Quick Facts
- Committee
- Judiciary
- Chamber
- House
- Policy
- Crime and Law Enforcement
- Introduced
- Jun 27, 2025
Referred to the House Committee on the Judiciary.
Jun 27, 2025
Official Sources
Official bill page with status, text, sponsors, and actions for the Undetectable Firearms Modernization Act.
The bill directly amends 18 U.S.C. 922(p), so the U.S. Code text is the core legal reference.
Official House-maintained U.S. Code page for the firearms chapter containing section 922.
The bill refers to detection devices commonly used at airports for security screening, which TSA administers.
Official TSA guidance on firearms in airport screening context helps explain the bill's focus on screened transportation settings.
ATF is the federal agency most closely associated with firearms regulation and enforcement relevant to section 922.
The bill preserves exemptions for licensed manufacturers and licensed importers under U.S. contracts.
PDF version of the official U.S. Code section containing the undetectable firearms provision the bill updates.
H.R. 4221 Bill Text
“To modernize the Undetectable Firearms Act of 1988.”
Source: U.S. Government Publishing Office
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