Quite a fair bit of the damage done to a service firearm tends to be excessive 'cleaning'. For example I have met idiots who felt it was a good idea to take a screwdriver and scrape the crown of the rifle until it shined. They called it the 'silver ring' (this being taught to them by other idiots who insisted upon this 'cleaning' standard). Other things such as constant assembly and disassembly (in particular over assembly and disassembly, such as trying to take of the pistol grip on a C7 which messes up the whole trigger mech) and abuse/lack of caring/lack of knowledge about the firearms leads to them being damaged.
Not sure about the Stens, but it still happens to this day and I suspect it happens in most militaries.