I don't know what the problem is with this forum, but about 75 percent of what I type gets tossed. If I try to edit a post like the one above, the edited version gets trashed.
What I'm trying (once again) to say is that all my characters train to 100 percent in repair skill. When they are done the game reports the weapon as being at 100 percent performance. It does NOT say that the gun's at 30 percent of full power, but that's as good as it can get.
Modern firearms, packed in cosmoline, and stored in a dry area, can be uncrated after centuries of storage, cleaned, and fired as if they were brand new. Millions of these weapons, stored in thousands of locations, are spread across the US. At least a million could be expected to survive a nuclear war. Since the population of the US could be expected to decline dramatically after a nuke fight, there should be at least one "like new" weapon per survivor. Guns--made mostly of high strength steel--are much more durable than people.
As an example look at civil war reenactors. Many of them fire weapons that were actually used in the civil war. The weapons function as well now as they did then.
I have a gewher 98, Erfurt 1918, so it's about 90 years old, and apparently not reworked since then. Although in excellent shootable condition, there is some use marks on the stock, and there are some rust pitting on some of the non action metal parts. This is a bolt action service rifle, 100% wood or metal. There has been a bit of torque on the stock (I like to think someone grabbed it by the barrel and swung it like a club, or maybe broke their fall with it, etc) so that it is not as accurate as it was originally, but it is still serviceable.
I have an M1 carbine, made by Inland in 1943, purchased by my father and given me for a present. A real war vet with signs of usage, but flawless operation. Again, a !00% wood/steel weapon.
Now, these two weapons, if correctly stored, might survive to serve well in a Fallout world, and with reasonable maintenance, could last quite a long time. Compare that to a modern weapon like an AR15, with it's plastic stock and forearm, fragile gas tube, unsupported barrel. It is fundementally easy to trash an M16 in combat, and although the action is simple, high tolerances require substantial maintenance and cleaning. it is, therefore, improbable that a weapons such as that would survive usage in a Fallout world for long. AK style weapons might serve better. The kind of firearms found in a FO world would likely be bolt actions, break open shotguns, simple blowback style submachinge guns...STEN and M3 styles, which could be manufactured at home and are very easy to maintain/repair. Any weapons with plastic stocks likely wouldn't survive. Weapons with complicated actions, weapons with high rates of fire (burned out barrels) and any weapons requiring strong springs, small parts, gas tubes/rings wouldn't likely last long. Weapons requiring LINKED BELTS wouldn't last long simpley because of alck of links, which, although easily fabricated, need to be fabricated with precision. In fact, any weapon using re3cuvled GAS PRESSURE would likely not be useful, du to deterioration of ammo supplies and the inprecision of home manufactured ammunition.
One must also consider that simple maintenance supplies...brushes and patches, and especially OIL might be hard to come by.
Again, this would be a realistic approach, and not necessarily useful in a game, which is supposed to be fun.