[Apache6]

Weapons used by low skilled personnel should wear out at a much faster rate. I'd suggest that a weapons skill roll should be a huge part of the equation on getting/keeping a weapon in action. That being said most issue weapons should still be functional if they have received normal wear (not having been found in a bomb crater where it sat for six months...).

M-16s, G-3s or FN FALs, and AK-47s if maintained should not wear out in ten years. Magazines can be a issues but I've got twenty year old rifle magazines that work fine. I've seen Ak-47s, Iranian G-3s and other weapons captured in caches that were covered with rust, but still functioned.

I've got a 1903 30-06 springfield bolt action rifle. My specific rifle was manufactured in 1928 and was purchased as surplus around 1950. It has been used for hunting and shooting fairly regularly since then, it gets cleaned after its shot and wiped down with oil every 6 months or so. I'm confident it would be reliable in combat. Bolt action rifles if maintained will not wear out with normal use.

Machine guns are a little different. Skilled machine gunners will employ their weapons in short bursts and change the barrel regularly, allowing the weapon and barrel to cool. Untrained personnel are less likely to properly clean and lubricate the weapon (and if applicable set the headspace and timing) and much more likely to fire the weapon at the cyclic (maximum) rate without changing barrels. This will shoot out the barrel, the barrels on certain weapons specifically including the M-60E3 can get so hot they will slump until the rounds punch through the barrel.