Why would anyone get excited about the .280 RCBS improved when we've standardized on the AI? It's a decent cartridge, but no one should be getting into an RCBS unless it's at a serious discount at this point. And no, the shoulder angle doesn't determine if it feeds well in a bolt action. Either feeds fine.
You have to be pretty concerned about recoil to prefer a .280 to a 28 Nosler. At short range it doesn't make much difference, but for the western hunter it sure does.
