I have a 257 Weatherby 24" tube with a 7.5 twist throated to shoot the 131 blackjack bullets. I am getting nearly 3300 fps with H1000 with the blackjacks and 3250 with the Berger 135. I think you will get better velocity if you build a rifle that will allow you to seat the bullets out farther and get more powder in the case. My buddy built the same rifle but put a 26" tube on his and he is hitting 3350 with the Blackjack. My COAL is 3.575 and I am running 72g H1000. I hope that helps.