I was thinking, shotgun hulls could be craftable. Paper cartridges are a craftable thing IRL, paper shotgun shells are a thing people craft. Shotguns are low-pressure firearms, they have thin barrels, weak actions. So it’s why shotgn shells are made from thin plastic. The brass/aluminum/steel head on a shotgun shell is literally only there to provide a sturdy lip for the extractor to grab.
I think shotgun shells are literally the only kind of modern-type ammo a survivor could hand fabricate artisan style. So post-cataclysm shotgun shell hulls would be 4-8 papers, superglue, tin can OR aluminum can OR scrap metal OR plastic chunks
Would require swage and die, cutting tool, hammering tool and maybe a stupid amount of time because hand-crafting shotgun shell hulls would be tedious and labor-intensive. Would not require precise machining like typical pistol and rifle cartridges, so it should be doable.