1. Have the chamber checked by the same smith and tell him what your seeing. Take some of that spent brass so he can see it.
2. If it is determined that the chamber is the problem but the bore is still good, consider having it reamed to 8mm/06, an easy conversion that allows you to load from easily available 30/06 brass. If you can find a smith that already owns the reamer it won' t cost very much. Certainly lway less than a new barrell job.

I hope you can figure out the problem without modification, b/c the 8X57 round is a fine cartridge.