Its really simple if you think about it...If you have a .50cal Muzzle Loading riffle then the barrel is a .50cal. You may shoot any projectile that is a .50cal. When shooting sabots, as elauwit explained, the sabot is .50cal diameter (well usually slightly smaller for easy loading) and the bullet that fits into the sabot depends on the inside diameter of the sabot...usually you will find a .45cal bullet paired with .50cal sabots. But there are others if you look hard enough...I have some .40cal bullets that have a sabot for a .50cal.
Just think of it like this...Your barrel has a certain diameter hole (in this case it's .50cal) What ever projectile you choose must fit into the hole but also fit snug enough to create a seal for the gasses caused by the burning powder. The sabot fills this diameter and the bullet fills the sabot...So, your sabot must be a .50cal and the bullet must fit snuggly into the sabot and should fit down to the base of the bullet with no air gap. It depends on the sabot as to which cal. bullet will fit. Most Muzzle Loading bullets now a days come with the proper sabot in the same package. :wink: