Since the topic of ammo has come up, I think that taking time, possibly in another thread (and possibly on another forum actually, not sure how well a detailed discussion on making ammo would go over here...) to talk about what it would take to make 'important things' in the kind of game scenario a given game might be using, such as making more ammo after civilization has pretty much ended.
So I would have to say that the hardest part about producing your own ammo is going to be your chemistry, not your physical dimensions. Making a bullet that is sized to a given firearm is really fairly trivial once you're set up for it, and even forming brass casings isn't that hard of a job, and it can all be done with just hand tools if you're really pressed for it. Reliable primers seem to be one of the tricky things, seeing as it isn't a well known technology even among people who deal with firearms.
I don't even own a gun, but I can make a moderately reliable basic black powder simply because I'm a history geek, and I know enough that I could probably cobble together nitrocellulose for a more modern product. (Oddly enough I learned about making that mostly from exploring early photography rather than weapons.) However making a reliable primer? That's rather tricky from what I can see. One of the biggest problems is that the stuff, by nature of its use, needs to be easy to set off, which in turn makes it rather tricky to produce due to how easily you can screw up and do unpleasant things like blowing your fingers off, or worse.
With a bit of googling it appears that lots of people seem to know a fair bit about the making of old black powder, far fewer know much about producing smokeless powder, and fewer still seem to have much to say about the subject of making primers or percussion caps. So that can make producing ammo a rather interesting problem, and could possibly cause tech to split in two directions. Some communities might fall back to using blackpowder in modern style cases with a weird revision on a flint lock and flash pan firing method, and other communities with better chemists producing modern smokeless powder with modern primer caps.
Thinking a little more about the problem and the risk of poor powder resulting in under velocity rounds is leading me toward the idea that a break barrel rifle might have a lot of value due to being able to quickly check that your barrels actually cleared before reloading.
To me, small details make a huge difference in game lore.