The big difference are the throat dimensions of the chamberings. The .260 has a nice short throat to fit a short action. But the .257 has a long throat to fit the 100-117 gr. round nose bullets that were loaded in it when it first came out. Seat a 100 gr. spitzer bullet out to the lands and you need a 3.0-3.3 magazine box and action lenth to accomindate the round.
Last I heard, Ruger makes their .257 Bob's in their long action just to allow this practice. E
