the 12 twist will do a B/C of about .28. I've shot Hornaday and Sierra 60 grain bullets. I got 5/8th's inch groups with virtually no load development. On the other hand I can shoot .30" groups with 55 grain Vmax bullets that have a B/C of .25. With a ten twist barrel, I think the 60 grain bullets would have been well under a half inch.
gary