Undetectable Firearms Modernization Act

Undetectable Firearms Modernization Act - Amends the Undetectable Firearms Act of 1988 to: (1) delay the repeal date for 10 years; and (2) extend coverage of, and exemptions under, the Act to undetectable 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 a magazine when subjected to inspection by airport x-ray machines.

Allows barium sulfate or other compounds to be used in the fabrication of a firearm receiver.

Defines a "Receiver Security Exemplar" and a "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 receivers or magazines that were previously prohibited but that become as detectable as the respective Exemplar in view of advances in weapons detection technology.