The 280 Ross was based on the 303 Magnum, another oddball cartridge. The head size is similar to nothing else that is available. Dominion loaded it until the 1960s or 1970s, so you may find a few cases at a gun show. You could buy base-stripping dies and make it from 450 NE or 416 Rigby brass, but that involves a lot of expense and a lot of work. Any way you look at it, shooting that old beast will not be an easy undertaking. Unless you opt for rebarrelling it, then you can choose your own chambering.