Better plastic guns can be built from blocks of plastic and hand tools then can be built using 3D printers. Why are they not freaking out about that. All these laws do is trip up good people exploring the capabilities of new technology. They do not address the issue of people wanting to use guns on others. We need to be addressing the issue of hate not method of gun construction.
To play Devil's Advocate...
The manual way requires a little bit of skill + knowledge + effort to make a plastic gun yourself. I'm well-educated, and even I would probably need some weeks / months to learn how to do everything the *right* way without fearing of making a mistake that will cost me a finger or an eye when it goes off.
This allows anyone that either has a 3D printer, knows someone that has a 3D printer, or knows where to steal a 3D printer... make one that's pretty reliable. And as the years go on, 3D printers will be more common. So you slip in a plastic gun past the metal detector, hide the pin and the bullet in a "pen" or something, and now a concert or high school or whatever has a danger inside.
AGAIN, just playing devil's advocate.