While I am a huge .257-caliber fan, the better choice among the listed cartridges are those using .264" bullets. While the diameters are only 0.007" apart, the bullets available for the .264s make the difference. A 140-grain .264" bullet will out-perform a similar 120-grain .257" bullet - no argument, it is just simply physics. If you need to hunt animals larger than the average deer, then the .264" bullets are the
better choice.
Since the OP desires a "mountain rifle" then the .257 and the 6.5x55 might be ruled out - they cannot be successfully chambered in a light, short action rifle. That leaves the .260 as the "best" choice for him.
While hardly a "mountain rifle" my current .260 wears a 26" factory barrel and shoots all bullets very very well. :wink: SPeaking of twist and light bullets, the best light bullets in my M700 are the 95 Hornady and the 100 Nosler BTip; both give similar accuracy (aggregates of ca. 0.6 moa) and downrange ballistics, although the Hornady is softer and probably safer for varmint use. Field accuracy among any of the three cartridges has
nothing to do with the case - it's
all in the gun and the care in loading.
.