Does the 600 OK use it's own propriety case or can this be made from another brass donor? I hadn't even thought about .620 cal - that's huge. If it could be done on easily sourced brass that could be the go. I can get Nyati brass from bertram or convert nitro brass that's why I liked the idea of 577 cal. Has any body looked at the 550 Magnum? Weatherby brass necked straight to 550 cal - easy to source brass...just a problem with the limited projectiles.