I made a chamber cast and cast of the bore at the muzzle end.
The chamber measures out at 2.75" with a slight shoulder, and the rifling grooves measure out at .369"
The rim is roughly .050" which is close to the Purdey version of the 400/360.
The 400/360 Purdey also had a bit greater diameter bullet - .367" from what I have read.

Maybe a sweet nitro for black, duplex or black powder load would be what is needed to get her up and running again.