if its a german made gun and the cartridge is a 450 BPE base you have a double rifle in 11,6x60R 450
https://naboje.org/node/4233

or 11,6x65R 450, similar but 5 mm longer case

notice the german version of the british BPE cases is not conical but cylindrical and then conical over the rim

and yes, basic case is the 450 express