An iffy 3d-printed gun using crappy materials can technically work for a few hundred rounds doesn't mean much in a country full of lots of legal guns of all sorts. And while I don't want TdA mass-producing RPGs, but that's not a problem solvable by regulating milling machines.
The problem is some people want total, invasive, inconsistent control over what other people can do, make, and be for ideological reasons rather than rational ones.
It's a metal tube that has some plastic around it to make it comfortable to hold. They are basically banning the production of "comfort features", not weapons themselves.
https://en.wikipedia.org/wiki/FGC-9
(The law is still silly, but for other reasons.)