Many ML rifle users actually use bullets not just ball; nothing to stop you firing a recognised expanding bullet in a muzzle loader.
eg. https://www.hornady.com/muzzleloadin...-xtp-bullet#!/
https://www.hornady.com/muzzleloadin...85-gr-hb-hp#!/
https://www.midwayusa.com/bullets-an...s/br?cid=23214