Re: Post-WW3 campaign idea
Certain ammunition types will be easier to remake than others. The 12-gauge shotgun is what I would consider the most prevalent and useful ammunition type in a post-apocalyptic environment. Pump shotguns are reliable, forgiving, and easy to maintain, every police department has them, and shotgun shells are easy to refill with powder and rocks. And it's the most popular type of ammunition for reloading, so it's easier to find a reloading bench.
After 12-gauge, I'd expect the most common ammunition types to be 5.56mm (for M-16s), 7.62mm (for older military rifles and some hunting rifles), and then pistol calibers in 9mm, .38, and .45.