After reading about the preban letter that Dave_G posted, I decided to take a look at the law. So I when to GPO's page and looked up the USC. Specifically Title 18, Part I, Chapter 44, which deals with Firearm Crimes. Being the masochist that I am([;)]) I decided to read the whole thing(that being Section 922). In my readings I found that the Undetectible Firearms Act of 1988 expires 15 years after its enactment. The UFA88 deals with firearms that can be seen on a X-Ray scanner. This is the feel good law passed after the polymer gun scare(aka the Glock). Anyway it will expire on November 30th, 2003.

Here is the act in full[its Section 922, subsection p; cited as 922(p)]
[i]
(p)(1) It shall be unlawful for any person to manufacture, import, sell, ship, deliver, possess, transfer, or receive any firearm--
>>>>>>>>(A) that, after removal of grips, stocks, and magazines, is not as detectable as the Security Exemplar, by walk-through metal detectors calibrated and operated to detect the Security Exemplar; or
>>>>>>>>(B) any major component of which, when subjected to inspection by the types of x-ray machines commonly used at airports, does not generate an image that accurately depicts the shape of the component. Barium sulfate or other compounds may be used in the fabrication of the component.
>>>>(2) For purposes of this subsection--
>>>>>>>>(A) the term ``firearm'' does not include the frame or receiver of any such weapon;
>>>>>>>>(B) the term ``major component'' means, with respect to a firearm, the barrel, the slide or cylinder, or the frame or receiver of the firearm; and
>>>>>>>>(C) the term ``Security Exemplar'' means an object, to be fabricated at the direction of the Secretary, that is--
>>>>>>>>>>>>(i) constructed of, during the 12-month period beginning on the date of the enactment of this subsection, 3.7 ounces of material type 17-4 PH stainless steel in a shape resembling a handgun; and
>>>>>>>>>>>>(ii) suitable for testing and calibrating metal detectors:    Provided, however, That at the close of such 12-month period, and at appropriate times thereafter the Secretary shall promulgate regulations to permit the manufacture, importation, sale, shipment, delivery, possession, transfer, or receipt of firearms previously prohibited under this subparagraph that are as detectable as a ``Security Exemplar'' which contains 3.7 ounces of material type 17- 4 PH stainless steel, in a shape resembling a handgun, or such lesser amount as is detectable in view of advances in state-of-the-art developments in weapons detection technology.
>>>>(3) Under such rules and regulations as the Secretary shall prescribe, this subsection shall not apply to the manufacture, possession, transfer, receipt, shipment, or delivery of a firearm by a licensed manufacturer or any person acting pursuant to a contract with a licensed manufacturer, for the purpose of examining and testing such firearm to determine whether paragraph (1) applies to such firearm. The Secretary shall ensure that rules and regulations adopted pursuant to this paragraph do not impair the manufacture of prototype firearms or the development of new technology.
>>>>(4) The Secretary shall permit the conditional importation of a firearm by a licensed importer or licensed manufacturer, for examination and testing to determine whether or not the unconditional importation of such firearm would violate this subsection.
>>>>(5) This subsection shall not apply to any firearm which--
>>>>>>>>(A) has been certified by the Secretary of Defense or the Director of Central Intelligence, after consultation with the Secretary and the Administrator of the Federal Aviation Administration, as necessary for military or intelligence applications; and
>>>>>>>>(B) is manufactured for and sold exclusively to military or intelligence agencies of the United States.
>>>>(6) This subsection shall not apply with respect to any firearm manufactured in, imported into, or possessed in the United States before the date of the enactment of the Undetectable Firearms Act of 1988.[/i]

Here is the sunset provision:
[i]
``(1) Effective date.--This Act and the amendments made by this Act [amending this section and sections 924 and 925 of this title and enacting provisions set out as notes under this section, section 921 of this title, and section 1356 of former Title 49, Transportation] shall take effect on the 30th day beginning after the date of the enactment of this Act [Nov. 10, 1988].
____``(2) Sunset.--Effective 15 years after the effective date of this Act--
________``(A) subsection (p) of section 922 of title 18, United States Code, is hereby repealed;
________``(B) subsection (f) of section 924 of such title is hereby repealed and subsections (g) and (h) of such section are hereby redesignated as subsections (f) and (g), respectively;
________``(C) subsection (f) of section 925 of such title is hereby repealed;
________``(D) section 924(a)(1) of such title is amended by striking `,(c), or (f)' and inserting in lieu thereof `or (c)'; and
________``(E) section 925(a) of such title is amended by striking `, except for provisions relating to firearms subject to the prohibitions of section 922(p),'.[/i]
FYI, 924(f)-penalties relating to 922(p)
925(f)-relief from disablities relating to 922(p)

I wonder if "The Secretary" ever constructed the "exemplar" mentioned.  Doubt it.

There are guns as well as knives that can't be seen on x-ray.  Not many and highly specialized to be sure but a few do exist.
Let me be the first to say I'm nit-picking because I am.  (The knives are far more readily available than guns.)

If a gun is undetectable, how would you know?