Re: Smaller capacity 6.5 or 6mm.
I have a 6x45mm with an 8-twist, 20" barrel (remainder of a 243Win barrel). Shoots 107SMK's at 2600fps, which almost exactly matches my 308Win dope. I built it for my kids and it's like shooting a loud 22lr that happens to shoot out to 800yds.
Just one trip through the resizing die makes the cases, so it's no trouble. But if I had an old fast twist .224 barrel laying around it would have built a .223 instead. No regrets either way.