Most common continental calibers are still in use today. 9,3x72; 6,5x58R; 6,5x57R; 6,5x52R are all in use with factory ammunition. Even 5,6x52R Savage and 30/30 Winchester were popular in Europe. My drilling third barrel is 6,5x52R, I use 25-35Win cartridges because they are fully compatible. If casting shows a more rarer caliber I would ream the chamber to one with same bore and of similar pressure.