I have read of 90gr SMK in 7 twist, but I played it safe and got a 6.5 for my attempt at a .223 FTR.
The Valkyrie is standard with a 7 twist but I have read of some disappointments with it and the heaviest bullets.
Code Name: JET STREAM
As noted it is the length of the bullet, more than the mass. We had good luck with the old 87 gr PRL's in 1/7 twists.
Facts matter...Feelings Can Lie