Yeah, that’s true. If electronics on Earth were fried, the civilization could have a tougher time coming back to life, but as Cataclysm’s verse stands, just setting up and rebooting everything seems perfectly viable.
Personally I think that a Metro 2033 type situation, where high-grade ammunition is essentially a currency, due to its rarity and impossibility of manufacturing more. I would be perfectly okay with having to eventually settle for pneumatic, electric, or paper cartridge weapons, at worst shabby and made out of few pipes, at best assembled from parts of existing guns.
Anyway, I researched ammunition manufacture extensively, and well, unfortunately, it seems like paper cartridges are the best bet for a survivor. Reloading brass cartridges is viable few times per cartridge perhaps, and primers are a serious problem - you could make some yourself, but not the kind that is used in existing ammo. I guess we could stretch things a bit here and say “you just can”, I’m more or less fine with that.
Also, gunpowder - we should make a distinction between black powder and modern propellants - gunpowder has essentially half the power, produces tons of smoke, and soots the inside of the gun - that would almost certainly interfere with the operation of modern firearms, and likely be unusable for any gas-operated system, which means no semi-auto or full-auto guns, just revolvers, shotguns, bolt-action, etc. Note that it would be usable, it would just break fast.
Regarding the rebalance/overwork of handmade guns, I really, really want to add disassembling existing guns and converting between calibers(within some limits, it’s perfectly viable for the most part). It needs some thinking though, because as of right now it could get a bit messy. An ideal solution would be a modular system, but that would probably be a huge and unnecessary mess. Either way, I’ll research and come up with what kinds of simplified part list gun disassembly could use and report back.