I've been loading for a long time and thought I'd seen it ALL. Guess not!
I built a long-range rifle for our local competition, "Long-Range Varmint Silhouette."
We use life-size, steel-cutouts of squirrels, marmots, crows, jackrabbits, coyote, and bobcat, at extended ranges.
The jack and coyote are at 500 meters and the higher classes of shooter has the coyote at 750 yards, and the highest class substitutes the bobcat at 850 yards!
Eventhough the shooting is prone with either a "pack-rest" or bipod, and any caliber, ammo, any sight(no .50 BMG, please) it's not an easy match.
I chose to build a .264 WinMag with 156gr Sierra target boat-tail bullets.
At first, things were fine, but as I got into the are that I thought I'd have an advantage, there were fewer holes in my target paper, and the paper is 48"x36"! I worked up my loads at 100 yards.
My scope-choice is a 20x Vortex.
Besides the fewer holes, some were slightly elongated. The REAL puzzler was that SOME were the shape of boomerangs! (I think you here call them bananas)
I found out some paper-shooters had the same problem, and it is "over-spin."
I dropped my velocities back to Grendel-Creedmore speeds, and the accuracy fell in line.
Now I have a Creed that uses 55-60gr of powder!!
My guess is that your 6.5 is a lot faster than my .264 and that's why you had trouble with an 1:8 and mine is 1:7.
Mine isn't nitrided, has no brake, and is a Shilen.
I CAN get acceptable accuracy at the higher velocities with lighter Hornady ELD-X and -M bullets, but I wanted high-BC bullets.
Have fun,
Gene