I had trouble finding any info other than the cartridge is in the Model 1876 "Centennial" Winchester. There is a 45-60 Peabody round also, but they are not interchangeable.
Cartridges of the World, lists the FL velocity as 1326 fps, which is typical of Back Powder. No Ideal bullet mold number was listed, so I can't even tell you what weight bullet to use.
Midway lists RCBS & Redding dies and shell holders. Brass can be formed from 45-70, but modern brass won't hold as much BP!
Somebody seems to be making ammo as Cimarron is reproducing the 1876 rifles.
Ivan