A Marble Gamegetter (or it's more modern equivalent...Springfield pistol version of the M6 Scout) The M6 is an over-under .22/.410 single shot. I read that the pistol version could also shoot .45LC.
I have the M6 rifle/shotgun and it is about the most vesatile gun I've ever seen. I can take anything from chipmunks (.22 shorts) to medium deer (slugs)with it. I guess loaded with CCI Stingers in the upper and buckshot in the lower it could serve as a threat stopper at close ranges too.
If ammo were to start getting scarce from some catastrophy.....
.22's are probably the most available cartridge in the States. .410, not so much, but any single shot shotgun can be loaded in a muzzleloader fashion if you have saved some spent high brass, primers, some black powder and lead. This can also be done with a minimum of equipment in the field.