Re: 6.8 SPC vs. 6.5 Grendel vs. .260 Rem?
I've got both the 6.8 SPC in an AR15 and the .260 Remington in a DPMS LR-260. Both good guns but the 6.8 can't hold a candle to the 260 ballistically. For that matter niether can the 308. I built the 6.8 as a deer rifle before they made the 22 center fires legal for deer hunting in TN. It works great for our whitetails out to about 300 yards. After that it runs out of steam for my way of thinking. The 260, on the other hand does well out to 500 yards and beyond. I've never had any experience with the Grendal so I can't help you there.
Now if you're shooting people, any one of the three guns will work. Humans are a lot easier to put down than deer.