This is probably old news to you guys but I found it to be good reading.
In the Jan/Feb issue of Petersons Rifle Shooter Jon Sundra used a necked down 7mm Rem. SAUM case to .257 with only the necks having to be reduced in thickness to make it work.
He built the gun on a Prairie Gun Works action, Shaw Fluted Stainless Barrel and a Boyd,s JRS Clasic stock.
I Quote " Thus far the results have been beyond my expectations.Working primarily with various 100-grain bullets, I've achieved 3500 fps with two different powders Re 19 and Re 22." out of a 24 inch barrel.
Not Bad. It goes on to say that they built a second gun just to make sure the results were not skewed. They posted groups .875, 1., 1.2, 1.25
What velocity would be expected from this with a 26inch or better barrell. Weatherby says 3600 from a 26 inch barrel in there 257 weatherby mag.