25 years ago, hunting eastern Oregon for mulies and antelope, I decided to build the "ultimate" long range rifle for medium sized game. Looking at BCs, SDs and case capabilities, I arrived at the .30-338, shooting the 200 and 210 gr Sierras at, or close to 3k FPS, from a 26" barrel. When it was done, I proudly took it to my 200 yd range, and began doing load workup.
I found a load that would produce the mystical/magical/(mythical?) single ragged hole group at 200 yds. The problem being, that by the time I got the rifle and load to that level (my expectations) the recoil had induced a flinch that took me the rest of the summer to overcome, using my .270 w/130gr sierras. 4,000ft/lbs of ME produces a heckuva kick!
Using a muzzle break was out of the question for me, because I value my hearing, and no tech was available at the time that allowed me to protect my ears and still hear hunting sounds.
Now along come the 6.5s, with similar BCs and SDs, that can be pushed easily to the 3kFPS level, and you don't understand why?? I would say recoil vs ballistics is the answer. The .260, especially in A.I. config is capable of matching most of the 300s and .338s ballistically, with the exception of energy on impact. Since penetration is largely dependent on velocity and SD, when bullet construction is the same across caliber choices, I say shoot the H377 out of that .260 until you get the accuracy/consistency you need, and go kill something.
And at the end of the day, you can put those ice cubes in your bourbon glass instead of on your shoulder!