Re: Long range caliber opinion
I favor the .260 over the 6.5CM because of multiple sources for brass, as well as the ease with which 7-08 and 243 brass can be converted to .260. Otherwise I see them as being in a dead heat, performance-wise.
Two of my preferred cases which were also omitted are the .30-'06 and the .280 Rem. The .30-'05 and its offspring .280 are excellent 30 cal and 7mm choices for 1000yd; and the .30-'06 is about the baseline maximum case capacity I'd be favoring for competitive use at distance. The .30-'06 is fully competitive, and the .280 will do anything it can do better, with either/both being readily available as factory brass.
While neither the .308 nor .30-'06 case designs are ideal by contemporarily vogue BR short/fat aspect ratio dicta, they have the advantages of longevity, good/broad performance load comprehension, and loading die availability. If you can't find a compatible and LR efficient bullet for the case designs, I'd merely suggest you look harder; they're there.
Maybe (maybe not...) oddly, I have <span style="font-style: italic">none</span> of the chamberings in the poll's selections among my rifles. That's not a criticism; it's simply a reflection of my current preferences.
Greg