I re-barreled a Savage Model 16, in factory 22-250 Rem,
with a 6mm Rem, which is the same parent case, as the 257
Roberts, and have absolutely no issues with a COL, where
the bullet is in the lands, fitting in the magazine. I have
played with the idea of getting a 257 Bob barrel, for it,
but I have a really sweet Tikka M695, in 25-06 Rem, and
at the moment don't feel the need for another .257 caliber
rifle. But I don't believe you will have any problems
with a 257 Roberts barrel, in the Savage short action.
You may want to order a barrel chambered with a shorter throat,
just in case the 120 gr. bullets get a bit tight in the
magazine.
Squeeze