I've played with both the 6.5CM and the .260Rem in the AR-10 platform. IMHO, I would give the nod to the 6.5CM over the .260 in the gas gun. Even slow, I could never get the .260Rem to print the groups I wanted, but it was easy with the 6.5CM, the factory Hornady 6.5CM ammo is actually amazingly accurate as well. Some of that has to do with the points already brought up about bullet weight. I have a bolt gun in .260 that I have shot Pronghorn, deer, and coyotes with. My 2013 Pronghorn was a 680 yard shot, with the suppressor. Watching the reaction of the doe to the death of the buck was very interesting.