The only usage of ammo_foo_obscure seems to be mod content, meaning that obscure ammo only drops with guns, or in areas using legacy itemgroups. To fix this, I suggest adding a variant of the antique store dedicated to old weapons, or adding a few spawns in json/mapgen/gunsmith.json .
Wait obscure ammo only spawned in the guns in question? No wonder I couldn’t find any 7.62x39s.
ammo_obscure has been added to gunsmith