The machinery that makes firearms isn't unique or special. Some of the tooling may be, but I can't imagine all the CNC mills an lathes are purpose specific at firearms manufacturers' plants.
As with most things firearms related, the offense occurs with the action. In this case, the transaction. You can manufacture a firearm with standard machinery and suffer no penalty. The same applies for a printer capable of printing a firearm (or parts). It is when you sell those firearms (or parts) that you have broken the law.
If it is against the law to possess a firearm in your location, then it doesn't matter how you made it. Possession remains illegal.
I see your point, but I am more thinking if 3D printers became household item then the cost, time, and specialized knowledge prerequisites that exist currently to make your own gun are all gone.
