Rob,
I would never say not to slug the barrel and/or make a chamber cast of any rifle. However, Axel's assessment is always well founded and I believe the rifle is chambered for 8.15x46R. Also I believe the groove diameter will be found to be .311-.313". We do, sometimes, see 8.15x46R chambers that have been extended to 8.15x52R or sometimes 32 Win. Spec. Firing a moderately loaded 8.15x46R in the rifle would likely be safe and would tell the tale as to the chamber. Slugging the barrel would be necessary to find the groove diameter. With the likely tight barrel, the fired case would be useful in deciding which bullet to use. The commercial jacketed bullet is shown as .324", this diameter is across a "stop ring", however, and the bullet is designed to be safe in various diameter barrels. Most people use cast bullets around .318-.319" diameter. Which ever diameter bullet will slip into the above fired case can be used in the rifle, despite having a tight barrel.
Mike