Still feel I somehow need to go back to first principles on this one ... I've personally handled weapons that were manufactured decades ago — back in the 60's, 40's, heck, back in the 1900's — and have ever since been in constant military use, passing through hundreds of hands, firing countless — tens, hundreds of — thousands of rounds, if not literal millions. Granted, they were well cared for that whole time, but my point is, these weapons worked as smoothly as ... well, some mildly obscene biological simile ... with no perceptible undue wear. Moving parts are well polished, but that's about it.
The only case where I've felt it was a bit ... questionable, was a particular sample of Pistol m/40 — a notoriously inferior copy of the Lahti pistol, manufactured under license in Sweden. But those pistols also suffered from being asked to fire the m/39B round, with its high chamber pressure ... something they were not originally built to do.
Other than that, they've all looked as if they were perfectly good for another century or two of hard use.
Point being, I have difficulty envisioning how military-grade firearms after a measly three years of war would be anywhere near a point where they're about to break down for good. Especially if they've spent those three years in the hands of military personnel who understand the value of treating your life insurance nice.
Sure, I've seen weapons break down to a point where they became non-operational — most spectacularly a couple of cases where people didn't understand that you're supposed to pull the M16A2's charging handle straight back and then release it, and consequently bent it (a "breakdown" that just as easily could occur with a brand-spanking-new weapon straight from the factory as with the one both your father and grandfather used, by the way) — but none of it was worse than that you could fix it with either the right spare parts or a semi-competent gunsmith.
So, how do you guys rationalise the system? Do you just say that what the heck, it adds flavour and suspense to the game so we just roll with it, or do you find some way to explain it that makes it sound plausible?