the caliber of your double rifle is 24/33 Lancaster Kugel

weight of the 24 ga bullet was 33,3 gramm, 18,7 mm long
it have 3 rings, upper ing measure 15,8 mm, the lower rings measure 15,7mm
sometimes called stop ring bullet because the upper ring stop the bullet on the case mouth when loaded with a hand tool.

just cut 24 ga Magtech brass to 33 mm and find a bullet mould. you can also start with a roundball but thats not so easy like the longer cases (40 mm long). the short cases were made for a load without a wad, just powder and bullet like any other rifle cartridge.

btw, nice little double rifle, would like to have it!