I have a CZ 550 American in 6.5x55. The rate of twist is 1:9. All I have ever shot through it is PRVI 139 grain. The rifle shoots this ammo very accurately. I am planning on trying some other ammo that is higher velocity in 140 grain. I have read that the 1:9 twist in my rifle won't stabilize 140 grain bullets. It seems to do well with the PRVI though. What are your thoughts?