Help writing a story, need people with knowledge of guns and bullets?

You can make your own bullets. You can even make your own gunpowder. But brass and primers require some manufacturing facilities that a collapsed government might not be able to support. Once all the factories were shut down, us gun nuts would be reloading our ammo until the supply of brass and primers was restored, and if it never happened, our materials would be good for only a finite number of uses.

This wouldn't stop people from using black powder guns (true historic models like flintlocks or matchlocks, not state-of-the-art T/C modulars which again depend on modern primers and non-corrosive propellent like Pyrodex). However, black powder arms are not nearly as speedy or reliable, nor as accurate as metal cartridge weapons, so their suitability for combat is limited. You'd have to come up with a reason why their use would be strongly discouraged. Like, the noise attracts the zombies or something...