It's actually a lot more insidious than that. According to the GOA:
Unless it existed before December 10, 1988, the plastic gun ban absolutely bans any gun that is not as detectable in a "walk-through metal detector" as a Security Exemplar (18 U.S.C. 922(p)(1)(A) and (6)).
The “Security Exemplar” is a piece of metal that the ATF uses to calibrate how much steel a manufacturer needs to put in the gun to make it beep in the metal detector. Other than the fact that it has to contain 3.7 ounces of steel and look sort of like a gun, anti-gun Attorney General Eric Holder can determine, by regulatory fiat, the characteristics of the Exemplar.
He can determine whether you test guns with a "top flight" metal detector -- or a crummy one. He can determine how many times (or thousands of times) a gun has to pass in order not to be banned.
In addition, every "major component" of every firearm has to pass through an airport x-ray in such a way that its shape is "accurately" depicted (18 U.S.C. 922(p)(1)(B)).
The problem is that the language of the law is so amazingly vague that the BATFE could use it to outlaw just about any gun currently on the market if they so chose.