Because an over and under has firing pins that strike a cartridge in the center of the barrels. I assumed you could swap the barrels to ones that don’t have breech plugs and be ready to go. Keep in mind, I’m not experienced with muzzleloaders like you guys are, so I don’t know if a muzzleloader o/u has hammers to strike a cap, or if it has pins that are inline and its cocked by opening the action. My Remington ml takes #11 primers, but has a bolt and an inline firing pin, so I guess I just answered my own question.
I’m just starting out, so please forgive my lack of experience.