FWIW (me not being MD) I've had no problems stabilizing 120s in a 1-10 twist, even with the slower .257 Roberts, however, one of the barrel makers, I forget which, recommends a 1-9" twist for 120s. Maybe Shilen.
My 120 use has been with flat based bullets ... partitions and hornady pointy hollow points. The longer boat tails might turn out different. If you did go with a 1-9" twist it might open the door to slightly heavier bullets from smaller bullet makers. Neither is a wrong choice.
Another option, if you have a lathe, is to shorten / lighten some X bullets so they would stabilize in your 1-14" twist. It's probably easier to change barrels but for completeness ... gotta mention it.
Tom
