I'll agree that metal is probably a requirement for a barrel. But barrels are not controlled or restricted. Neither are magazines, which we know can be printed already with some success.
It's also worth pointing out you can already buy all of the components necessary to assemble your own firearm, all without a serial number. I think the deeper issue here is, as another comment pointed out, there are entrenched interests promoting this hysteria to garner public sentiment for regulation by making the scary applications glaringly obvious and overshadowing the beneficial implications of the technology.
Not exactly true. Every legal gun has one part with a serial number that defines the gun legally. Usually (always?) it's the receiver. There are some grey areas where you can get an almost finished receiver free of regulation.