Re: .260 Rem vs 6.5-08
You can look up the reamer specs at places like Clymer's.
IIRC, the biggest difference is that the 6.5-08 had its origin in slow-fire, single-load competitive venues like metallic silhouette, NRA High Power, etc. As such, guns chambered in 6.5-08, from what I've seen (had one, and know a number of people who have others) tend to have longer throats to get bullets like the 142gr SMK seated out further in order to get the body/heel junction of the boat-tail past the shoulder/neck junction aka 'donut city' on the case. Yes, HP shooters do a fair bit @ 200/300 w/ mag-length loads, but in my experience the bullets used at those distances (120gr SMK, for instance) can take a poop load of jump and still shred the X-ring.
The .260 Rem was intended by Remington strictly as a magazine-length hunting round, and the bullets are seated deeper accordingly. There's no reason you *can't* cut the throat longer on a .260, or shorter on a 6.5-08, but that is the fundamental difference as far as I know.