So it's okay to 3D print a gun as long as you have a serial number? That seems to reinforce that 3D printing shouldn't be banned, especially by blanket technical means.

That is the essence of the weird gun laws. Take a Glock pistol as an example. The only part that has a serial number and is legally “the gun” is the thing you hold in your hand: the frame. It’s plastic and has the trigger and some parts to hold the magazine.

The rest of the stuff? You can buy and overnight ship it to yourself legally with almost no regulation (as of 2026 CA requires more gun-like treatment for those parts).