I would say it is a 25-20 Single Shot, based on the physical appearance of your casting. The 25-25 and 25-21 were straight cased cartridges, much longer that the 25-20 SS. From accounts I have read, the 25-20 SS was a common caliber for re-chambering these types of rifles when proper ammunition became unavailable. If it is a 25-20 SS, Buffalo Arms can help you out with dies and brass. Good Luck!

GMC(SW) - USN, Retired (1978-2001)