I've been using 75gr & 77gr out of both a 1:7 & 1:8 AR's, I'm sure at full velocity you could stabilize a 70gr with even less twist. I have an older tang safety M77 in .22-250 that shoots the 64gr Nosler BSB really well. I'm not sure what twist rate it is (slower anyway) but, I'd say work up a load near may and see if they shoot. If they make round holes close try them again at whatever distance you'd be willing to use them at.
“You never need fear a man, no matter what his size. When danger threatens, call on me, and I will equalize.”
Samuel Colt.
�Common sense is genius dressed up in work clothes.� - Ralph Waldo Emerson
| 
