As they said in the movie Planet of the Apes: where there's one, there's another, and another, and another...
Like the drug trafficking networks.
If that is what's happening in Australia imagine the scale it would take place in the US. They do not even have as nearly as many civilian gun companies. With many civilian gun companies going out of business and many workers who have seen and know where to get blueprints, knows how to use CNC and proper heat treating equipment out of work, we won't just see improvised firearms we would see knock offs of existing handguns, assault rifles (Glock, Smith&Wessons, Colts, H&Ks, Walther, Izhmash, Beretta etc), probably some black market custom brands or maybe even RPGs and grenades.
Metal and material? Heck I know some rich people who would love to supply them e.g Mafias, well connected street gangs and probably even legitimate metal companies. Remember during alcohol prohibition there were some wealthy and influential people in on alcohol production and smuggling and the authorities turned a blind eye to it.
Equipment? Again, I know some rich people would would like to give to black market gun makers equipment. Any company working with metal has them. Heck most of it is available in basic machine shops. The 3d Printers also might come into it eventually if the Black Marketeers refine it to the point of making it work.
Smuggling? We would also see another market open for criminals south of the US border e.g. The Cartels. The Cartels are well know for smuggling in guns and ammunition from China Russia, and even out from armories from very unscrupulous elements in the Mexican police and military.
Some people will say that but most people won't be able to find them blah blah blah. Uh people the kids in the Broncs know where every street corner drug dealers are. It is really arrogant to say that they can't find street corner arms dealers or illegal factories/workshops.
Just my take on it