If you go with your assumption (US jurisdiction), it can indeed be fully automatic with the correct tax stamp.

Manufacturing a firearm with a 3D printer would be a federal crime in the USA. That's not why I asked.

I'm more interested in actual costs of the democratization of force for places that don't already have weapons (unlike the USA).

Interesting - as a private citizen, I was under the impression that I could not own a fully-auto assault rifle. I'd be interested in hearing sources that point to the contrary.

Great point about the democratization of force. I wonder if it's any easier than obtaining them on the black market in those countries.

With a 3d printer, it becomes not "those countries" but "every country, simultaneously".

PS: "fully-auto assault rifle" is redundant.

Actually, it only has to be semi-automatic to be considered an assault rifle.