thats simply great! one of the very early big bore dangerous game rifles. I dare to say it will need 8 bore pinfire cases and there is no other ( military) case available you can use to make it. maybe the older 20x110 Hispano Suiza with 24,5 mm base diameter could be lathe turned.
https://sallyantiques.co.uk/product/brit...eenwood-batley/
not the 20x110 US navy, that another bigger case.