Printing technology becomes cheaper and eventually into most homes like regular computers and printers banning guns will be useless. These are un traceable and can be created for afew dollars in material. Examples on the internet show guns that can be assembled with 3D printed lowers and non serial number parts of amazon anyone could buy that can shoot over 100 rounds.
Digital file import or export is very easy and can be replicated infinite number of times.
Banning guns is already useless, criminals buy guns off of the streets, not stores. Plastic may be cheap, but a 3D printer is not, and your standard criminal is not a very wealthy person. 3D printed guns wouldn't be an issue in the U.S. at least, firearms can be carried for self defense. We may even see a decrease in crime due to increased accessibility to firearms.