I think it's stupider than that. They have no idea how a handmade gun could be built without a 3d printer. They probably have zero clue what a 3d printer actually can and cannot do, and I'd bet most politicians have never seen one or bothered to understand it before regulating it. Their sole exposure is a few loud people who also do not understand anything about guns or 3d printers, and confuse that lack of understanding with definitive proof that it is evil and should be banned.

Same principle applies to the frequent attempts to ban semi-auto rifles, when rifles account for 3% or less of all homicides annually.

Those kind of weapons are effective for defending / attacking a moderate sized area, unlike pistols and bolt-action rifles. Pistols are short range and bolt-action rifles are slow. It's obviously about the power that they don't want the people to have.