OP,,,do NOT cut/chop/modify that gun in any way,,,,,,,try to use modern day Lapua 220 Russian ctgs,,,,I would turn the neck on one ctg to .010" and try it in your chamber (no powder or bullet )(you may have to trim the length,,,check everyting) if it goes then I would load this one ctg with a bullet and check the seating depth,,,,then fire that one ctg and see how far the neck expands ,,,that will give you the neck dia,,,,then you can turn the necks (if required) to the proper dims,,,,,proceed slowly and carefully,,,,,Roger
PS ,,,,DO NOT MODIFY that rifle,,,,