The same rule would apply to printed guns.
That is also my understanding.
As 3D printed guns become mainstream, oppressive governments will initially shift their concentration to restricting ammunition while scrambling to control 3D technology and materials.
Sort of sounds familiar.
Not quite true if my understanding is correct. You can still sell your homemade firearm, you just can’t be in the business of making them for sale if you don’t have a license. Seems like you’d be going out on a limb liability wise though. You sell it, someone abuses it and it blows up, they sue you and claim it was defective.
What would particularly complicate things would be a set of 3d printed parts which could be combined with widely-available products and materials which are not associated with firearms. Most parts of a typical firearm will be subjected to severe acceleration, but not to much stress other than that resulting from their own inertia. Making a few high-stress parts from metal would allow the rest of a firearm to be made from lower-quality materials without an excessive increase in size or weight.