keith,
To start with, the groove diameter of many of these barrels was larger than .318", usually about .320-.321. This was intentionel,being the first german military rifle for a smokeless powrer/jacketed bullet load,they still had black powder barrel provisions on their minds. I don't know if it was to allow room for powder residue, or to allow room for the bullet jacket to expand when it is engraved by the rifling( of course we now prefer bullets the same size as groove diameter).The 8X57IS cartridge was not introduced for a couple years after it was approved.This time was used to inspect all the rifles in the German inventory and modify them as necessary for the new ammo.Sights had to be modified to match the trajectory of the new ammunition.More importantly for this conversation, during testing, it was determined that a slightly oversize bullet doesn't always excessively raise the pressure.On the other hand, if the neck is jammed into the leade area of the chamber,and can't expand to release the bullet, then pressure does rise excessively. As part of the checking inventory to insure safe use of IS ammo, the tight chambers were reamed to essentially 8x57IS dimensions.They could, therefore, be safely used with the new ammo.However( and this is a big "however"), this inspection/modification was not necessarily done to rifles not in the German military inventory.Excluded from this inspection were 8x57I chambered rifles sold to other nations, of given to allies.Also,civilian sporting rifles didn't have this inspection(some may be OK and others not). Do I have you confused enough yet?
Mike
