take a .589 barrel and chamber for a 50mm long 28 ga brass case loaded with blackpowder and bullet.
it was a original husqvarna cartridge in the M 17A combination gun but I have also seen pics of rolling block stalking rifles with full stock.
the 28ga Magtech brass is cheap and hold looooong with black powder. similar to the 577 snider that will kill anything in north america.