Considering you can make a "zip gun" with a piece of metal pipe and something to smack the firing pin on the bullet, I don't think a plastic gun is really going to be all that attractive in the long run, especially with the fact that the bullets get quite hot when they fire and you risk melting your newly printed gun (or having the barrel explode in your face). Right now people are interested in it, just because it's a novel idea, not because it is at all practical.