ORIGINAL: cayugad
Speer ... look on the side of the bulk carton the bullets came in. If it says .451 or .452 diameter and they are a .45 caliber then get some Harvester (black in color) .45 caliber crushed rib sabots. Those will fit the Speer and you can shoot them out of the .50 caliber rifle.
He will have to get some sabots to fit44 cal if the Speer hollow points are .44 diameter or .40 diameter, as you know he can getsabots from MMP OR I believe it is best to buy packagebullets and sabots for a 50 cal muzzleloader, since he is so new at this ML stuff. Buying the package boxes of sabots and bullets is the best thing to do and it is best to buy them at a Sporting goods store that sells MLer rather than Walmart who have folks that are clueless about MLers.
Newbyhunter, can you take a picture of the bullet box where it says the size of the bullets and post it?
With the proper info we can help you correctly, I am feeling because your so new to this stuff that what we are saying is too much info and it is confusing to you, especially on bullet diameter and sabot sizing.
Chap