Filaman,
Yes, the 1:10 .257 will handle 120s very well.
About the longest 120-grain made is the Nosler Partition, at 1.175 inches. Have never had any trouble getting them to shoot well in any 1:10 .257 Roberts.
You might want to try using a good twist/stability program before assuming so much. The Berger program gives the 120 Partition at 2900 fps a stability rating of 1.42 in standard conditions (59 degrees F. at sea level), which is pretty tough for stabilizing. At higher elevations, such as my part of Montana (4000' minimum) the rating is 1.56 at 35 degrees. At a rating of 1.0 bullets will stabilize, but 1.5 is considered VERY stable. Obviously the 120 Partition is stable in a 1:10 in almost any conditions.
The 115 Barnes is a little longer, and has a lower BC of .335. But even then it's stable under standard conditions, and at 35 F. and 4000' gets a rating of 1.33.
Over the past 2-3 years I've had a hard time understanding understand why so many people assume that the standard 1:10 twist in .25-caliber rifles isn't enough for standard .25 caliber bullets these days. But believe I am finally getting it: Apparently 1:8 is the Magic Rifling Twist for any cartridge these days, no matter the caliber. Well, here's the deal: A "fast" (or "slow") twist is in proportion to the caliber of the rifle, the bore/groove diameter. Thus a larger caliber doesn't require as fast a twist to stabilize even long, heavy bullets, the reason the standard 1:10 twist in .30 calibers will stabilize a very long 210-230 grain spitzer boattail. It's also why the standard 1:14 twist in 9.3mm will stabilize even 286-grain monolithic bullets. Which is also why a 1:7 (or 1:6 or 1:5.5) twist is necessary in .224-caliber rifles to stabilize many of today's heavier, longer bullets.
Also, one thing you might discover when using such programs (which have proven to be very accurate) velocity has FAR less effect on bullet stability than most shooters believe.
