I built a 6.5SS with as skinny of barrel as possible while still threaded for a brake/suppressor. I did a #4 mullerworks 1:8 twist. Spiral flutes and had one inch cut off the breech end to reduce as much weight as possible, and finished at 24".
Barrel is right at 3lbs. Has 9/16"-24 muzzle threads that fits a TBAC brake mounted ultra7.
Shoots slower than id like, but accuracy is great. 156's at 2830fps. 3-shot groups (I don't let it cool between shots), and it has been reliably under 1/2". I'd not hesitate to get another skinny mullerworks, and I no longer pine over getting carbon barrels.
A couple weekends ago I zeroed the rifle at 100 (3/8moa at the paper), then cooled the rifle, then shot this to check dope at 530