338 I think Roy Weatherby wanted to sell rifles with a proprietary cartridge. I guess 416 Rigby cases were the largest practical at the time so he added a belt. JOC used to turn the belts off 416 Weatherby brass and neck it down to make 416 Rigby cases. I guess 416 brass was pretty scarce back then.
My understanding is the Creedmoor was designed as a match cartridge so the case shape was designed to feed correctly. Pretty sure the shoulder angle is 30 degrees.
