Tinker,

Some close ups of the relevant markings are below. It'll be a while before I have time to do a chamber cast but a quick measure shows that the depth of the chamber to the rifling is about 1.235" and the diametre of the chamber at the breech is .473" or almost exactly 12mm. The max rim diametre is approximatly 0.550" but its a strange shape and hard to measure , kind of countersunk in on an angle rather than a square shaped groove as found on more modern guns. The rifling has 6 shallow grooves with a RH twist. The lands are narrower than the grooves. Looks to me like typical Enfield type rifling, but I'm no expert. The muzzle diametre is very close to 0.400" or 10mm.

Any help is appreciated.

Regards RJW