Raimey,
I have Vernier calipers, dial calipers, electronic calipers, Vernier micrometers and electronic micrometers, but none of them will measure to the "millionth of a mm". When I broadcast Rye, I use grain rye, the deer like it better. Since you know better than the owner ( or his father that left him the guns) what caliber it is, you can tell him. My assessment is the double rifle was proofed in Suhl between 1911 and 1922. If that is wrong, it wouldn't be the first time. I am open to an explanation of where it is wrong, based on the info given.
Mike