I think because of the substantial difference between the shape of most current .257 bullets and any future high bc bullets, the throating would have to change on all the old SAAMI specs, otherwise the loaded cartridges will be running out of magazine length or alternatively running out of neck grip on the bullets because of the more radical boat-tails and pointy tips. So newly designed cartridge cases would be the way to get around this problem. This is the problem I faced, I couldn't get the throat right for both future .257 bullets and the current ones...so had to reluctantly go to 6mm.
