MMP 54x50 sabot with any of these

50cal DC 300 or 325gr. The 325gr is the bullet Knight shipped with their 54cals.

50cal Hornady 300gr FTX. These expand best at higher velocity and shoot flatter than most 50cals.

458s might work in the Harvest 54x45 sabot made for .452s. They work in mine but do load tight.