Just for kicks I'd say the Civil War era .44 Henry rimfire. You could buy it in bulk packs like you do .22LR right now and use it in big bore versions of the common rimfire platforms out there. It would be a fun short range plinking cartridge, like .22LR on crack. Imagine taking out tin cans with a .44cal, 200 grain bullet lazily lobbed from your new Ruger 10/44HR rifle.
