As 3D printers become common household items, the same level of gun control will require detailed mass surveillance and/or restrictions on common items like pipe and scrap steel.

Are people worried about un-riffled longarms though?

Unless you have a way to rifle the inside of a pipe, that won't really be an issue. You'll just be making inaccurate muskets.

The important item isn't the 3d printer. Its the CNC Lathe.