What is the Craft DC for making alchemical cartridges, such as the paper cartridge? I can't find anything listed in Ultimate Combat.
If you have the Gunsmithing Feat and a least 1 rank in Craft (Alchemy), there is no check involved; you are automatically successful in creating ammunition (at half the listed cost for alchemical cartridges).
If you don't have the feat, then you are correct that does not seem to be a listed crafting DC. Might I suggest an extrapolation from the core Craft skill list? DC 15 for crossbow bolts (or "high quality items"), or perhaps DC 18 if you consider it "exotic" (ala exotic weapons). Even DC 20 if you consider them complex or superior. I would probably stick to 15 myself, two-handed firearms users need those alch. cartridges.
Edit: Hmm, I totally missed the new item Craft DC's in the feat section of Ultimate Combat. Firearms are set at DC 20, and since in the Core weapon and ammunition are the same DC together, I guess I would go up to 20 for alchemical cartridges.