If sticking with 223 powder capacity, I think the 75-77 grain class is the sweet spot, and it doesn't require more than 8 twist (my 9 twist does fine as well). My 223 in 20" 9 twist hits 3" steel at 600 repeatedly in the wind with 75 ELD-M's. Not at 900 though - 1 hit every 5 shots or so on 12" steel. If you go to a bigger case, then perhaps twist it faster for 80-90 grainers, but I've read and know people at the range that have found the heavier pills finicky in both Valkyrie and 22 Creed. Better to move up to a 6mm if going that heavy IMHO.
