The amount of brass is certainly slim pickings, and since they dropped the steel case ammo in A17, I imagine that heavy gun use can really tear into your brass reserves. But mining brass wouldn't realistically be a thing, as it's an alloy of copper and tin. Perhaps if they added copper and tin to the mines, as a lesser item found during mining for lead and iron (not terribly realistic, but more so that mining brass), and then you could make the brass in a forge.