I'm not saying the availability of 3D print designs will suddenly make little jimmy that got a printer for xmas say to himself, "well now that I can print a gun, what kind of mischief can I get into." I don't believe that either. I agree that criminals will be criminals regardless of the law, 3D printers or not.
All I'm saying is it'll be almost impossible to regulate the printing of these things. When you build a gun now, don't you at some point have to buy a receiver and that requires you to go through the same process as buying a gun? (Serious question, I have no idea.)