And I'd argue that shell casings are probably harder to manufacture than a fully working firearm. The equipment needed to manufacture working ammunition end-to-end is pretty serious.
All of these manufacturing equipment and processes existed more than a century ago.
If you have a capable VMC, you can make the die and other equipment necessary to stamp shell casings from commonly-available parts and machinery.
From there, with a modern Dillon or Hornady reloading press, you can crank out thousands of rounds per day without issue.
Primers are a legitimately difficult thing to manufacture, but (good-enough) bullets, casings, etc. are completely doable.
All of these manufacturing equipment and processes existed more than a century ago.
If you have a capable VMC, you can make the die and other equipment necessary to stamp shell casings from commonly-available parts and machinery.
From there, with a modern Dillon or Hornady reloading press, you can crank out thousands of rounds per day without issue.
Primers are a legitimately difficult thing to manufacture, but (good-enough) bullets, casings, etc. are completely doable.