6 & Half Dozen! Shoulder of the CM theoretically gives a slight increase, although increased pressure and compressed powder loads to match 260 velocity, add to throat erosion..So.. its a wash. I’ve stuck with the 260 because I already had dies, load data & an endless supply of super cheap, LC brass. Anyway, if I was looking to switch up at this point, I’d go 260 Terminator, or perhaps a 6.5 SLR Like an “improved“ AI. 140gr at a solid 3000fps.
I continue telling people though....if you are a factory ammo shooter, go with 6.5CM. Can’t touch the selection. I’m a reloader. My 260 even has never eaten a single factory cartridge.