6.5x52R
The 6.5x52R is a European cartridge that is a close variant of the .25-35 Winchester. It should not be considered identical to the .25-35 Winchester and is not treated as such by the Commission Internationale Permanente pour l'Epreuve des Armes Feu Portatives (CIP), which has released separate specifications for each cartridge. The 6.5x52R has its origins in Germany and was introduced by RWS and is chambered in many drilling and single shot rifles. Any interchangeability between the two cartridges is solely dependent on individual chamber tolerances and should not be assumed
I suspect this may possibly be the cartridge. The only way to tell for sure is to get a chamber cast. Any good gunsmith can do this for you.
Jim
