john,send him your message as a pm
what I understand is that he shoots this beast. the cartridge must be very similar to the 50/110 Winchester in the original blackpowder load or the old german 12,7x65R 500 express. the short 500 BPE which was also available with a 60 mm long case (12,7x60R 500)is one of the rounds I am looking for a rifle seriously for many years now.
remember a belgian cape gun(true cape gun because the rifled barrel was on the left side)for the 50/110 Win Exp.
this 12,3x63R here must be realy cylindrical! the mauser case measure 13,00mm on the base so with a 12.3mm bullet diameter you have 0,35mm for the wall thickness.