There is an economy revolving around this in Russia and parts of Eastern Europe. Using history books, diaries, old maps, war stories from veterans, et al. you have a group of people that find "lost" weapons, ammunition, materiel and then clean and repair it. Then they sell it to gangs and the mafia.
Dig up a dozen PPsH submachineguns and you'll have enough parts to cannibalize and repair half of them into operation. Sell each one for $500 USD and you've now bought groceries, clothes and medicine for the year for your entire family.
The mafia likes buying these kinds of guns because when they use them, and discard them, they are traced back to the Soviet Union in the 1930's and 1940's. There's no dots to connect from a forensics standpoint.
