Dale,
Re: 6.5/308, for all practical purposes, I had Chris from Pac-Nor who built it using a no neck turn reamer, send me the drawing for the reamer, simply neck down 308 brass, and it is from what I can tell 99+ % the exact cartridge as the 260, with perhaps the wildcat being a very tad shorter, but I am not positive, when I was in the middle of the project, I heard rumors as Jim Carmichael was writing about the Panther and Bobcat 6.5's, and inklings Rem was going to intro the 6.5/308 as a commercial round. Lonoke, AR ammo plant sent me a saami spec drawing before the release of the 260, and it was as I said, as best as I could tell w/in VERY minute differences one and the same. I contacted them as Rem corporate would say nothing until I explained, then later said, cannot confirm nor deny. Lonoke was much more helpful.
That said, I went forward as I wanted to know if factory ammo would be fine in my gun, I later bought and shot some and it was. SO, that is it in a nutshell.
Any surprises? It did well, and best with WW 308 brass, Federal 308 down was too tight, Fed 243 necked up worked ok, but also perhaps a 'donut' at the neck shoulder junction. Many who reform brass for 6.5-308 and the 260 from 308 brass turn necks to help prevent or reduce donut effect which can prevent chambering IIRC.
I hope that helps. H414 and IMR 4350 did best, used 100-129s but the factory 140 and 140 partition handloads never grouped well in the 9 twist.
I prefer an 8 twist as Ruger did them, over Rem rifles 9 in the 260, most 6.5x55's have 8 (7.5 on mil surp) but a few have 9 twist. 8 works best for 130 and higher grain bullets. That custom was 23" .750 muzzle taper, shot well. Later had a SS model 7 20" and 18.5 blue and neither shot to my satisfaction, barrel seemed to whippy. Hence the SS gun being rebarreled and my blue being traded off.