It must be an optical illusion, but the photo showing the muzzles of your rifle makes the bores appear too large for 6.5mm, unless the barrels are unusually thin at the muzzle. What is the actual diameter of your barrels at the muzzle? I would think that they would have to be at least .464" (11.8mm).

Beautiful rifle, by the way! I once had a Westley Richards drop lock in 6.5X53R, but 6.5X57R is much more desirable, especially with the scope.