I'm glad you guys are able to come up with some good discussion and reasoning to fill some of the holes the writers overlooked. Who knows; maybe these ideas will find their way to the next generation of Fallout writers and we'll have helped make more believable canon.
I think one thing that goes unrecognized in the games is that there is a huge discrepancy between pre-war factory standards and post-apocalyptic availability of precision machines to produce weapons and munitions of the same quality.
[spoiler:fd81058134]I'm no gunsmith or firearms expert; I've just done a lot of reading on hand-loading and machining, and this is the opinion that I've gathered.
Weapons:
It is possible to find pre-war gun parts laying around separately in different storehouses, and with some understanding of firearms, be able to put together your own weapons in post-war conditions. Some components may even be able to be replicated through hand tools that may be available in post-apocalyptic conditions -- stocks, grips, and other non-precision parts.
I think that the real problem comes with extremely small tolerances involved in the parts that absorb the most force from discharging a modern firearm, which are also likely to receive the most wear during regular firing.
Modern barrels are typically rifled in order to provide a bullet with spin to add stability to its flight, and the inner diametres of the barrel has to be extremely close to the widest diametre of the bullet (we're talking precision in thousandths of inches or tenths of millimetres) or else it will provide a poor seal for the propellant gases. A bad seal will mean you'll lose a lot of energy behind your shot due to blowback, and have more unstable flight of your projectile. Even if you managed to get lucky enough to find an unused barrel made to pre-war specifications, and were able to clean and lubricate it properly and took good care of it, it would still degrade over time as you continued to fire it. Eventually you would start to lose potency from both the rifling and the seal, which would result in a drop in accuracy and damage, as well as increasing the potential for malfunctions. The machinery required to re-drill or re-rifle a barrel is quite substantial. If anyone's curious about just how much engineering and technology goes into it, this link should provide:
http://www.border-barrels.com/articles/bmart.htm
Other fine parts in the gun, like any number of mechanical components in the many different designs of receivers, may wear out. Even if you had access to a properly-maintained machine shop with the power supply to run its machines, along with the knowledge and skill required to operate the machines properly for your tasks, you'd have to know the exact dimensions of the original parts in order to create working replacements. This would require finding the blueprints or reverse-engineering the design to try and replicate a working stand-in part. The amount of technical skill, time, and experimentation would probably be significant and the quality of the finished product would be dependent on the availability of the materials used to create it.
Black powder weapons such as smooth-bore rifles, muskets,
Ammunition:
Different problems, but still dependent on quality. The main components of your modern cartridge are the casing (usually brass), the chemical propellant that it contains, the bullet that goes in the end of it, and the firing cap. Brass casings can be re-usable. The firing cap and the propellant will be burned up on firing, and the bullet will be deformed. The casing will be easily retrievable and if you could find a hand-loader or make one of your own somehow, you could make more ammunition from your spent casings. Eventually they will start to degrade and deform, but they should be common enough not to be the limiting factor in making a round of ammo.
Good bullets are actually very easy to make if you have a quality mould and bullet-sizer. You need some kind of material with a high density and low melting point, like lead. Lead can be found in lots and lots of different items, especially in the '50s before environmental regulations started to limit its use. Statues, models, pipe, weights, already-fired bullets that are too deformed to use on their own -- all of these things can be melted down in a simple pot over a heat source, the impurities skimmed off, and then poured into a bullet mould in order to cast your own bullets. The finished product will only be as good as the mould it's cast from, so you'll need a precision-made mould, or a really good bullet sizing tool to tune the dimensions a bit. If the bullet is too large by even a few of thousandths of an inch/hundredths of a millimetre, your bullet may get stuck in the barrel during firing -- if you fire the weapon again with a jammed barrel, you risk your gun blowing up in your face and possibly killing you. If your bullet is too small, you'll have a bad seal and get lots of gases escaping, giving you the same problem I explained earlier with barrel wear. Some effects on bullet dimensions and methods for sizing are discussed here:
http://www.lasc.us/Brennan_4-1_BulletSizingBumping.htm
Propellants are chemically manufactured with their chemicals mixed in specific ratios in order to get just the right amount of 'boom'. You could probably find containers of pre-war gunpowder laying around after the war, but it would be a non-renewable and rapidly-diminishing commodity. The rarity and price would continue to rise to the point where it might only be something priceless that you'd see closely guarded by heavily-armed organizations. If you can't find it or trade for it, you'd have to make your own propellants. Black powder is simple enough to make yourself, but it's typically only good for muzzle-loading smooth-bore firearms which have terrible accuracy compared to modern firearms that use rifled barrels and smokeless powder as a propellant. Black powder also gives off a lot of smoke and lines the barrel with soot. If you were to fill a modern round of ammunition with black powder instead of smokeless powder and fire it, it would just cause the gun to explode. Smokeless powder is much more chemically complex to make and would require specific knowledge and equipment to refine. It would also need to be made to just the right specifications to not cause firing it in a gun to damage the gun over time or risk misfire.
Firing caps have very similar problems, but they require their chemicals to be even more sensitive to detonation. You might be able to get away with simpler, easier-to-obtain chemicals, but they will still be the most dangerous part of the entire weapons/ammunition manufacturing process. Pre-war manufacturing works around this by having remotely-operable machines with heavy shielding between them and their machinists -- unless you could somehow come into possession of such a thing, you're going to have a very high chance of blowing off your fingers at the very least. You also need to worry about any corrosive chemicals eroding your barrel with every shot.
Adding a very small amount of extra smokeless powder to your shot has a very fine line between making it more powerful and causing your gun to explode. Black powder weaponry can typically handle venting a bit more over-pressure if you accidentally use a bit more black powder than you should have (you still run the risk, but it's smaller compared to smokeless powder). It also uses lead balls which require much less precision in their casting and don't require the assembly of a full cartridge.[/spoiler:fd81058134]
TL;DR: Guns and ammo will be less accurate, reliable, and damaging in the apocalypse until some sort of quality industrial conditions are restored. There would probably be a renaissance in black-powder weaponry and explosives, with more modern firearms gradually being limited over the centuries to groups that have the military force and/or scientific expertise to actually maintain them.