re: #3 lostlakehikerThe problem, I guess, would be the bullet. Anything dense enough to serve as a bullet will be dense enough to show up on an x-ray

You’re assuming bludgeoning damage — that the mass is needed. What if instead of a traditional bullet they fire sharp ammo — needles or arrowheads, of a style?

While more susceptible to body armor, a “gun” that fires what amounts to razor blades or other such shrapnel would be devastating to soft targets (flesh, for example) — and said shrapnel could be made thin enough that it wouldn’t appear on an xray.

Having said that, I think 3D Printing will be banned long before this particular use becomes mainstream — although this may be the justification they use to neuter this new technology.

Because if I can download an iPod and make it at my Fab@Home or RepRap station, why in the world would I buy a real one made by slave labor in China?

For reference, some home 3D Printer kits:
fabathome.org Fab(ricate) at Home, the first home 3D Printer project I know of.
reprap.org RepRap. Project Thesis: “Human Technology has Progressed to the point that a Von Neumann Self Replicating Machine is possible to create.” A project designed to make a 3D Printer capable of printing… Itself.
makerbot.com Makerbot: They make a basic kit that is very popular, and is spun off of the above two projects.