You're overstating the requirement; amateurly machined barrels work fine and that'll all you need for an FGC-9. The FGC-9 weldless bolt can be made by a monkey with an angle grinder. The FGC-9 was specifically designed with European regulations in mind. FGC-9's can be built by idiots using common garage tools, built out of commonly available materials. You cannot stop it with any degree of regulation.

You can restrict the availability of suitable ammunition. That will be the ultimate outcome if politicians continue to let guns scare them. It's a largely unfounded fear; there aren't many politicians getting shot with any kind of gun, let alone homemade ones. Even in America where professionally made guns are easy to get, politicians rarely get shot.