Those Rainier barrels you're speaking of are made by Black Hole weaponry, and are not Obermeyer 5R cut. They are more like a rifling with 5 waves or bumps. Look at their site (thier new site is almost worse than their last one) and google Polygonal rifling and you'll see http://en.wikipedia.org/wiki/Polygonal_rifling
Lothar Walther also makes polygonal barrels, but I think they are only 3R. Those, I believe were the barrels used for the original LaRue OBRs before LaRue started making their own barrels.
For top quality, you can get a .308 5R cut barrel blank in the contour of your choice from Bartlien or Krieger and have a gunsmith chamber it for you in any gas length system you like.