PGM Ultima-Ratio .308 Counter Sniper Rifle
There is a thin layer of oil between the barrel and the outer finned shell that helps dissipate heat when shooting. Great concept, but dammned heavy! This finned barrel type is seldom used on the newer FN PGMs (FN bought out PGM's design). FN has opted for the shorty "Commando" barrel type.
Picture from Birdman who is also the proud owner