Paul-

The rifle of which I speak is seen in my signature line.
I've written of it here and elsewhere here on NitroExpress.

The 3-1/2" cartridge (and the rifle) dates back to the early 1880's
It was a NA&A BPE proprietary round, originally of coiled brass construction. J&W Tolley were known to run this and other NA&A cartridges.
There's another member here Marrakai (from the NT), who also owns a J&W Tolley 500/450 with these chambers.

The original ammunition was loaded with either a ~285gr tubed bullet or a ~360gr soft round nose semi-jacketed bullet.

Cheers

--Tinker