Confusion here, Sir - muzzle-loaders do not have calibre designations like this, where the .45 refers to the bore size, and the 60 to the BP powder charge. You could, if you feel so inclined, totally fill a barrel with a powder charge - making it a .45-1000, for instance.....
The 45-60 is a black powder era Winchester cartridge , NOT a muzzle-loader.
It was designed for the Winchester 1876 Centennial model, and remained in production until 1935 or so.
It's pretty easy to make this cartridge from BELL .45-calibre cases made for the Sharps and any other .45 long case cartridge. I have all the details, if this is your intention.
tac