Will the advances in 3D printing gut future weapon legislation?

In another thread, someone alluded that so long as you use durable enough material a rifle, pistol, etc. could be manufactured with a 3D printer and have the same quality and tolerances of a piece fashioned the traditional way; in addition to that, there would be no serial numbers on the rifle, etc. Should the advances in 3D printing become that good (if it has not already) how would that gut future weapon or gun legislation in the future? Someone like David Cho who might be thwarted from purchasing a pistol if legislation somehow included backgrounds for psychiatric treatment, they can just 3D print up their own weapons, same with felons, or anyone else who would run into a roadblock purchasing a gun at a sports shop or gun show. If you do not know who is printing up their own weapons, how do you regulate it?