True, however not every part of a functional weapon needs to be milled metal. Hence the 3D printer AND a milling machine, although I have seen some smaller caliber, all plastic handguns that have been quite durable. As for ‘gangsters’, I’ve worked with thousands and trust me, many are very tech savey. This would be nothing for them to do when it becomes a necessity.
Zip guns aren’t anything new. Milling sounds decent for actual combat weapons. Combat weapons involve firing thousands of rounds out of the gun. AKs are available on a parts basis on the black market. If anyone really wanted an AK, they could get all the parts cheap. The ammo sounds as if it could be on the market too.
As an example, the stock and grip aren’t necessarily milled metal, although I am sure a lot of people. I generally prefer metal for the barrel, for taking the pressure and heat of repeated firing. (Marksmanship and hunting)