As I understand it, after a certain date Oberndorf Mauser sporters were all chambered to the same dimensions, irrespective of the barrel bore and groove diameter, which made them safe to shoot with either size bullet. If an 8X57IS round with the .323" bullet will chamber in your rifle and the bolt will close easily, it should be safe to shoot.
The easiest way to confirm this would be to fire an 8X57I (.318" bullet) through the bore and then check to see if a .323" bullet will easily pass into the mouth of the fired case. If it will, you have sufficient neck clearance and can use either bullet.
