I have the option of picking up a used rifle in the said chambering with a 9 twist benchmark barrel. I'll be at 800' elevation. What bullets do you think it will stabilize?
It will stabilize all but the long 1.35 long match 130's and 140's. It may stabilize those too but you never know until you try. Most are 1:8 or 1:8.5.
Mine is a 26" 1:8 Shaw that shoots awesome with the 140 Amax.
You might be able to spin the 140's. I have two 6.5-06 at 8 twist shooting 140 amax close to 3000fps. I have one 260 27" that will stabilize 140's and it is a 1in9.