Generally, the heavier you go with a bullet the faster twist you need. The 139-142 class bullets will need a 1:8 twist barrel. The lighter bullets generally need less twist because they have less bearing surface and have more velocity. So, if it can stabilize a 139-142 class bullet it is likely to have no problems stabilizing a the lighter bullets. The only problem you might run into with the lighter bullets will being able to load close to the lands. But rest assured, you will have zero problems stabilizing a 120grain bullet with a 1:8.5 or 1:8 twist. Just order a 1:8 twist for for peace of mind. For what its worth, I have never saw anybody get an aftermarket barrel for the 6.5x55 that wasn't an 8 twist. I have a shilen select match 8 twist on mine and love it. Hope this helps.
Justin
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