Well, I haven't said anything because I don't know the maker. It's a nice rifle and you will enjoy shooting it once you get it up and running.

My first thought was the same as kuduae: a .450-400 2 3/8" BPE aka 10.3 x60R. That's the one I would vote for. However, he makes a good point that there were other possibilities on the Continent during that period. I'm not familiar with any of the other Continental possibilities.

I believe we can surely rule out the .450/.400 3" as that cartridge came too late and was never loaded as a black powder round. It was a Jeffery development specifically for nitro loading with the intent that it not readily chamber in any black powder chamber.

The .450/.400 3 1/4" BPE is a possibility and would be an excellent outcome to this riddle. That is a wonderful cartridge and very pleasant to shoot. Yes it was later adapted to nitro loading, but clearly this is a black powder rifle, so don't even toy with that notion.

I have a marvelous James Woodward double rifle chambered in the .450/.400 3 1/4" BPE. It is a delight to load for and a joy to shoot. If that's your chamber, you are in luck; and if so, I'll be glad to share my loading data with you. I'm loading NfB in that one.

A chamber cast is imperative. You should be able to order cerrosafe for delivery to the UK. It's Brownell's product, but I think Midway USA may stock it and maybe also Huntingtons. It's silly simple to use and answers these riddles with authority. Contact Brownell and you problem in that regard should be easily solved.

Now as to the rifle itself, I have one complaint: no sling eyes. (Very minor complaint, I admit!) When you take this weapon afield to smite a deer you will have your hands full when dragging your deer out of the woods and carrying the rifle without provision for a sling. It's certainly a beauty and looks to be well-restored. A bit to "new" looking for a 100+ year old antique, but many like them restored.

You are really going to enjoy shooting it. Congratulations.

Curl