I marvel that a discussion on 3D printing centers on the singular topic of making guns.

The miracle of 3D printing is that it can make working models (copies) almost anything the mind can conceive. While I don't know about their long-term durability, just think of all the things one could make from simple gears and replacement parts for machinery to complete artificial hearts and other prosthetic devices...if not now, someday soon.

And, yes, with great technology comes great responsibility, to paraphrase Voltaire (I think). So you think negatively on guns, I'll look forward to medical technology.