My first “custom” varmint rifle was a 12” twist Hart on a Rem700 action in 6mm Ackley. It shot 55BT’s into tiny tiny groups at nearly 4300fps. It shot 70BT and Sierra BK great as well. I started wanting to shoot deer with it but never could get a100gr bullet to shoot under about 2” at 100 yards. It also shot 85 bthp sierras really well also. I’ve since shot quite a few deer with it with 70BTs and it kills like lightning. But if I were to do it again I’d go with a 10” twist. That barrel is on its last leg and when it goes back for its next barrel it will be a 10” twist. Probably another Hart but perhaps a Krieger.
