Joe,
Certainly you aren't suggesting that we "print" 30 round magazines, are you? That would be illegal, against the law. That would make us criminals. Can I help with the testing? Purely scientific investigation, no criminal intent!
When 3d printer are illegal only criminals will have them...
I can imagine that they would try to go after anyone who downloaded the design files for these evil things. Having knowledge will be illegal.
Actually, one use would be to make replacement parts for pmag style units so your old grandfathered units never wear out. Probably not as simple as that, but if they develop a method to print plastic around a metal part with a strong bond, you could incorporate feed lips or a rifled pistol barrel or other wear parts into designs. You could even incorporate duplicated registry number plates into duplicated guns so you have 10 disposables with the same number. How would anyone know if this was the real gun or a copy.
If history is a guide then the the real use will be something we have not thought of yet but likely even more annoying to the busy body control freaks who need to tell the rest of us not to do what they secretly know they would do if they had any balls!
