Here's a SxS 12 bore, shipped right to your door.

https://www.youtube.com/watch?v=ywR5M0MJVCA&ab_channel=TAOFLEDERMAUS

Please, do NOT use Pyrodex. Pyrodex is 17% by volume, perchlorates. Perchlorate fouling, when mixed with moisture (even just from the air) becomes acidic & has an affinity for iron molecules, according to the tests done by Bill Knight for Dan Pawlack, inventor of that 'replica' powder.