Hi TH44,
Maybe I should start a separate thread, but I wont be able to post any photos for a while. It seemed a coincidence that the dimensions of two unknown calibres in two separate countries should be so similar.
The bore diameter is .417" as near as I could measure, and the groove diam was .429". My thoughts were 400 Boxer, which I think was an earlier version of the 400Kynoch.
My dating of the rifle Was done from "British Gunmakers Volume 2", however the street address on the barrel was not listed. The serial number puts it in the 1860's.
If I buy it I will then put up some photos.
This is the reference I used for "bore dimensions" http://www.fourten.org.uk/shotgunbores.html
They match up pretty well to the dimensions from the chamber casting.
Regards
LesLeeSpeed