Undetectable Firearms Modernization Act

This bill amends federal law to update how "undetectable firearms" are defined under the current prohibition. It replaces outdated metal detector standards with a new magnetic field requirement (equivalent to 3.7 ounces of stainless steel) for detection, and clarifies what counts as a "major component" (e.g., frame, slide, or barrel for rifles). The changes primarily affect firearm manufacturers and importers by setting clearer detection standards for compliance, while exempting all firearms in U.S. government possession or under existing military contracts. The law does not create new restrictions but refines existing legal definitions for enforcement.