1 in 8" would allow you to use as heavy a bullet as you can get in a 6.5mm, such as 160-165, etc. However, I believe a 1/10" would stabilize a 140 PSPBT like the Sierra.
I will be building a 6.5-284 soon and would like to know what the best twist rate would be for 140Gr pills. I am leaning towards 1 in 8 but would like to hear some ideas.
My Hart 6.5/284 is 28 inches long and has a 1 in 9 twist rate and stablizes the 142's just fine at 3030 feet per sec. Accuracy amazes me, and at ranges out west to 1 mile the rifle is incredible. I want six more just like it!