A lot of people have large supplies of ammunition for their go to guns in case the SHTF occurs. And many of those same people also own various black powder guns they shoot for pleasure, some are used during muzzle loader season to extend hunting opportunities. you may not think about it but these are your ultimate back up guns because black powder can be home made much easier than smokeless. These may be modern inlines or reproduction percussion cap or flint lock firearms.
Although you have plenty of ammo for your main gun do you have enough 209 primers, percussion caps, and flints to keep your BP guns working? It would also be nice to have a round ball or minie ball mold in each caliber just in case.
A lot of people have cases of military surplus ammo for one firearm and only a tin or two of percussion caps for another. Cover all your bases.