The “premise” for the firearms was stated outright, that local junta leader had outlawed guns for all but his people. As for standard firearms brass doesn’t last forever, if you don’t have the power to run the machines to make new casings eventually you run out of brass to reload. They’re at pre-industrial revolution technology most of the time, at least when they have to make their own stuff.
I've read just the opposite on multiple firearms sites. If stored properly, such as a cool and dry environment, ammunition will last indefinitely.......