You can counterfeit money on traditional printers, but we didn't ban printers (not as serious as building a deadly weapon, but still a huge crime in our society). The current solution is that big-league photo editing programs, copiers, and some high-end printers have mandated built-in detection algorithms for currency[1].

Maybe the same thing will happen for weapons? That becomes a harder problem in 3D, but are there common patterns unique to a receiver or barrel assembly or something?

I don't love any mandated black-box code running on my devices, but it would be better than most of the alternatives I can think of. (Banning, doing nothing at all)

[1] e.g. http://en.wikipedia.org/wiki/EURion_constellation