Every few 10 years or so, BATF will have one of their phony baloney “amnesties” where they encourage folks who “somehow missed” all the previous amnesties for registration… sometimes, some really, really gorgeous pieces come out into the light of day… Fully appointed Maxim guns in their original caliber, complete with transit case, MTOE, sometimes early optical sights, tools, parts and manuals… and they have brass water jackets. Once they’re cleaned and polished up, holy moly! What a gorgeous piece of work those things are!
As far as anyone making what they want? Blueprints abound, all over the place. If you can do sums and work a metal file, you can make anything (of course, just using files, it will take a really, really, really, really long time… which is why machine shops will be popular). The holy grail, of course, would be an actual working example of what you want to make. Disassembling it, blueprinting each part, you can test the hardness of each piece to make sure your heat treat is right, see how everything works in harmony with everything else… that’s what the Afghanis did to the Russians – captured Russian weapons, took them apart, reverse engineered them, then built their own out of raw materials in small workshops… then they beat the Russians with their own stuff.
What man has made, man can make again.
The wicked flee when none pursueth..." - Proverbs 28:1