Should you 3d print an auto sear (a little bracket-looking thing) then you're in possession of a machine gun, which is illegal.

Why was this downvoted?

The federal laws do state that "possession of specific machine gun parts is considered possession of a machine gun"

Machine gun sears included. Also parts that convert a semi-automatic into a machine gun like a Lightening Link.

Because it's a non-sequitur.

Unregistered machine guns are illegal to possess regardless of how they were made.

3D printing an otherwise legal firearm is not illegal at the Federal level in the United States.

The exchange boils down to:

Comment: That isn't illegal.

Reply: well, if you do this other thing that's illegal, it is.