OSS,
Raimey is more familiar than I with Austrian proofs so I will defer to his opinion. As far as the nominal chambering is concerned, the Austrians commonly used a slightly different system of marking than the Germans. Where the Germans marked the barrels normally with the bore( not groove or bullet) diameter, the Austrians often used the groove diameter. This means the groove diameter of your rifle barrel should be around 8mm or .315".This is a little tight, but 8.1mm is .319" whereas we normally consider the 8mm sporting groove diameter as .318". Considering tolerances, we can likely disregard cartridges with .323" bullets. In addition to the common German cartridges, the Austrians also chambered rifles for their own cartridges. All this is a round about way to say you will likely need to make, or have made, a chamber cast to determine the nominal chambering of your rifle. Slugging the barrel will determine the actual groove diameter( or extend the chamber cast into the barrel). If the seller included cartridge cases fired in your rifle, they could be used instead.
Mike
