The unique twist ratio, I believe, is also available from custom barrel makers, like Hart, Douglas, etc. Douglas builds for the Military almost exclusively.
For the best .308 benchrester, I would cut the barrel to 19 & 1/4 inches, with a step-recessed crown. That would tighten my groups to under .250 center to center. Using fully accurized Bench Rest components, especially balanced and trued brass, a rifle like this is capable of competition standards of less than .100 accuracy for a 5 shot group.