The Undetectable Firearms Modernization Act aims to update the existing prohibition on undetectable firearms by revising key definitions and detection standards. It redefines an "undetectable firearm" by requiring that all parts, excluding major components, contain a specific "detectable material" that produces a magnetic field equivalent to 3.7 ounces of 17-4 PH stainless steel. The bill also provides a more precise definition of a "major component" to include the slide, cylinder, frame, receiver, and for rifles or shotguns, the barrel. Furthermore, it clarifies that the prohibition extends to prototype firearms and updates the criteria for how firearms are assessed by airport security detection devices. Finally, the legislation revises exemptions, removing the previous general detectability exemption and instead allowing for firearms received by or under contract with the United States government .