Undetectable Firearms Modernization Act

Undetectable Firearms Modernization Act - Amends the Undetectable Firearms Act of 1988 to: (1) delay the repeal date of such Act for 10 years, and (2) extend the prohibitions against undetectable firearms in such Act to specified firearm receivers and ammunition magazines.

Prohibits the manufacture, importation, sale, shipment, delivery, possession, transfer, or receipt of any receiver for a rifle or handgun, or of any ammunition magazine, that: (1) is manufactured by a person who is not a licensed manufacturer, (2) is not as detectable as the Receiver Security Exemplar or the Magazine Security Exemplar by walk-through metal detectors, or (3) does not generate an image that accurately depicts the shape of a receiver or an ammunition magazine when subjected to inspection by airport x-ray machines.

Defines "Receiver Security Exemplar" and "Magazine Security Exemplar" as objects fabricated at the direction of the Attorney General that are: (1) constructed, respectively, of 3.7 ounces of material type 17-4 PH stainless steel in a shape resembling the lower receiver for a rifle or handgun, or of 1 ounce of material type 17-4 PH stainless steel in a shape resembling an ammunition magazine; and (2) suitable for testing and calibrating metal detectors.

Directs the Attorney General to promulgate regulations to permit the manufacture, importation, sale, shipment, delivery, possession, transfer, or receipt of receivers or magazines that were previously prohibited but that become as detectable as their respective Exemplars in view of advances in weapons detection technology.