Bullets aren't really a problem, casings can get reused and as long as there's some softer metal like lead forming a seal for the rifling you can use not quite properly fitting rounds. You loose efficiency and get mechanical problems though.
More likely is that a gun lieing in the wasteland for 200 years has rusted beyond use. Even a decade should be enough to ruin the mainsprings or weld the reciever solid.
Sure t34's can be recovered from polish swamps 80 years later and be drivable after restoration, but the complex mechanical parts are mostly sealed in oil filled cases, also a swamp is actually a fairly good place for preservation, low oxygen levels etc. gun parts are small too and having tiny tolerances means microscopic corrosion is enough to break operation.
That guy with the AK full of Twinkie put his much abused glock into a dishwasher and it rusted immediately. To be honest all the corrosive shit he put that glock in probably affected the factory anti corrosion coating, but 200 years in the wasteland has got to be pretty bad.
Edit: why isn't any of that stuff buried? The earth does move, at the least you would expect roads to be buried and houses in worse condition, here's a random picture of a gun dug out of a hole after 100 years.
To be honest, the trench itself looks almost exactly like decayed fallout shack walls and junk fences, so good job there. Contains graphic images, there's a bone sticking out of a helmet and possibly more I missed.
http://www.dailymail.co.uk/news/article-2099187/Bodies-21-German-soldiers-buried-alive-WW1-trench-perfectly-preserved-94-years-later.html