Lighter bullets and frangible types (SX and Blitz) will disentigrate with the fast twist rifles. However, with the fast twist you can shoot the heavier 60+ gr bullets have great stability due to their ballistic coefficient. If your giong to shoot only light weight bullets go with the 1:12 but for those long range shots on dogs the heavier bullets are the way to go and the 1:9. You cannot sucessfully shoot the heavier bullets in a 1:12 rifle-they will not stabilize. Have fun