The United States Undetectable Firearms Act of 1988 (18 U.S.C. § 922(p)) makes it illegal to manufacture, import, sell, ship, deliver, possess, transfer, or receive any firearm that is not as detectable by walk-through metal detection as a security exemplar containing 3.7 oz (105 g) of steel, or any firearm with major components that do not generate an accurate image before standard airport imaging technology.
It was signed into law by President Ronald Reagan on November 10, 1988.
This summary is from Wikipedia.
The summary below was written by the Congressional Research Service, which is a nonpartisan division of the Library of Congress.
10/21/1988--Senate agreed to House amendment with amendment.
(Senate agreed to House amendment with an amendment) Undetectable Firearms Act of 1988 - Amends the Federal criminal code to make it unlawful to manufacture, import, sell, ship, deliver, possess, transfer, or receive any firearm: (1) which is not as detectable as the Security Exemplar (after the removal of grips, stocks, and magazines) by walk-through metal detectors calibrated and operated to detect the Exemplar; or (2) of which any major component, when subjected to inspection by x-ray machines commonly used at airports, does not generate an image that accurately depicts the shape of the component. Defines the term "Security Exemplar" to mean an object that is suitable for testing and calibrating metal detectors and is, during the 12-month period beginning on the date of enactment of this Act, constructed of 3.7 ounces of stainless steel in a shape resembling a handgun. Directs the Secretary of the Treasury, at the close of such 12-month period and at appropriate times thereafter, to promulgate regulations to permit the manufacture, importation, sale, shipment, delivery, possession, transfer, or receipt of firearms that are as detectable as a security exemplar which contains 3.7 ounces of stainless steel or such lesser amount as is detectable in view of advances in state-of-the-art developments in weapons detection technology. States that no provision of this Act shall not apply to: (1) the manufacture, possession, transfer, receipt, shipment, or delivery of a firearm by a licensed manufacturer for the purpose of examining and testing such firearm to determine whether it would be prohibited by this Act; and (2) any firearm which has been certified by the Secretary of Defense or the Director of Central Intelligence as necessary for military or intelligence applications and is manufactured for and sold exclusively to military or intelligence agencies of the United States. Permits the conditional importation of firearms for the purpose of examination and testing to determine whether the importation of such firearms will be allowed under this Act. Provides an exemption from such prohibition for any firearm possessed in the United States before the enactment of this Act. Provides criminal penalties for violations of this Act. Prohibits the Secretary from authorizing the importation of undetectable firearms. Directs the Administrator of the Federal Aviation Administration to conduct research to improve the effectiveness of airport security metal detectors and airport security x-ray systems. Directs the Attorney General, the Secretary, and the Secretary of Transportation to conduct studies to identify available equipment capable of detecting the Security Exemplar while distinguishing innocuous metal objects. Repeals such prohibition ten years after the effective date of this Act.