ammo is much harder to manufacture
I’m not sure if that’s true. I don’t have any experience with it myself, but lead is notoriously easy to cast at home, because of its low melting point.
So it seems to me, all it takes to make some (shitty) ammo is some lead, a mould (which you can make from clay) and an explosive.
For a proper modern gun, that last piece is probably pretty difficult, and you would need to make casings somehow too (I’m not sure how difficult that would be). But if you were using some form of (possibly homemade) flintlock or similar, the gunpowder from fireworks should be enough.
I think regulating both guns and ammo is the way to go.


Technically not one book, but uhh… one/two book series: Mistborn by Brandon Sanderson.