Did you know that it is often legal to make your own firearm in a 3D printer? These printers generally create plastic components, and people will download plans off of the internet and then print the guns at home. There are many cases in which you can do this without violating the law.
However, one stipulation is that the gun has to contain at least some metal components for it to be legal. If it is entirely made out of plastic, that is not allowed. Why is it that the presence of metal itself is such a key point in whether or not this firearm is legal?
The general idea here is that a lot of places use metal detectors for security. Airports, for instance, have metal detectors that people have to walk through before going on the plane. The same is true for some government buildings or schools. Though metal detection systems will also trigger for things like knives, the main focus in modern America is to find people with guns.
As such, if people were allowed to legally print plastic guns without any metal parts, they could get around these safety systems. That is considered to be a massive risk to the public, so all guns need to have some amount of metal in them so that they will set off these alarms. This isn't to say that you couldn't make a gun without metallic components that would function, but it would be illegal for you to do so and you could face very strict ramifications if you were caught.
If you do find yourself facing weapons charges of any kind, make sure that you know about all of your criminal defense options.