I think that your post is at least partially tongue in cheek, but "undetectable", especially when in quotes, isn't intended literally. It almost certainly refers to the nomenclature of the "Undetectable Firearms Act of 1988" or subsequent renewals.
The language of the actual US Code (18 U.S.C. § 922(p)) uses much more specific and technical language:
http://trac.syr.edu/laws/18/18USC00922.html
...
(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 Attorney General, 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 Attorney General 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 Attorney General
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 Attorney General 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 Attorney General 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
Attorney General 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.
...
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