There are alot of 257's. My favorite is .257 STW
You can push a 100 gr bullet 3950 fps .Most people are building them on a Rem 700 action . The barrels range from 26 to 30 " . I was told by a well known smith who builds alot of these that it should be no shorter that a 28" barrel to get the speed it deserves.
Mine that I am building is a Rem stainless 700 , 28" fluted lija barrel
stainless, Mcmilain stock and a muzzle brake.
My scope will most likely be a Nightforce 5.5 x 22.