The .45 caliber could be used in the muzzleloader as a conical if the diameter was right on the bullet. It might not be, but I am not sure of the accuracy you would get. You might also get a lot of leading in the barrel...

Ypu could shoot powerbelts in .45 caliber or they shoot sabots using .40 caliber or .357 caliber projectiles. I do not own a .45 caliber but have shot a few of them. I would stick with full sized concials because you have reduced the size of the diameter enough just going down to .45 caliber in the first place.

If you read your manual I am sure they suggest some bullet combinations in there that would work for you.. Good luck with your rifle.