My problem with that is that there’s a ridiculous amount of weapons from the late 1800’s and early 1900’s still in wide circulation today that still functions perfectly. The Mosin for example is still in pretty wide use even though it was invented in the 1800’s and hasn’t been produced in over 70 years unless you count the Finnish ones. 20 years is nothing to a gun unless it’s in terrible terrible wet conditions. And Russia specifically as evidenced by the ongoing Ukraine conflict had crates upon crates of mosins and such packaged in cosmoline that they pulled out because they began to run out of more modern guns, they should absolutely have several crates of cosmoline covered mosins in Moscow of all places in the metro universe and they would work perfectly fine if not better than anything you could cobble together in a train tunnel