Depends what was the intended use for the law. It's not always about enforcing it on the spot. Take another example - declaring you never joined a war on some visa applications. Of course they're not going to check that. But if it ever comes up and is discovered for some (un)related reason, you're out without further discussion.

Same can be applied here. Found to have one during some checks? You're charged.

Did Lethal Weapon have plastic firearms? My first thought was In the Line of Fire.

I'm sure there was some genuine fear, e.g. I'm thinking about NY Republican gun grabber Peter King, who after fellow Representative Gabby Giffords was shot (with a Glock...) proposed a law that would create a floating, 1,000 foot bubble around important people like him in which it would be illegal for normal people like us to possess a gun (!).

But like what this discussion is about, there's a strong pattern by the serious gun grabbers to try to ban anything new, be it the first wildly successful polymer frame handgun (the Glock of course has a steel slide and barrel, but the original proposal was to ban it despite it being easy to detect), or these newfangled printed guns.

How is that?

Care to explain why you feel this way?