I have made a few ARs in 260 with 20" barrels and they do just fine. I would not worry at all.
Now to address the rabbit trail:
Mobuck says FWIW
The outfitter I hunted elk with last week won't even let his hunters uncase a 6.5C after too many failures.
I too know of of way too many failures on elk with the 6.5 C but I know what the problem is, and it's not the shell. It's the bullets that some of these guys insist on using.
They seem to think a bullet that shoots 1/2 MOA is "better" for big game than one that shoots 1 MOA, and never think about the facts that (A) elk are big targets and (B) any elk bullet needs to be able to break a heavy bone and still penetrate at least 24 inches after it does.
I know of quite a few elk killed with 243s and 257 Roberts, neither of which are more powerful than the 6.5C, and those elk were killed wonderfully. But the hunters used bullets that hold together. My daughter is one of them.
Even the old 6.5X56 Mannlicher is just fine for elk and and it's not as fast as the 6.5 C, but the old M/S was usually used with 156 or 160 grain bullets that did exactly what I described above. expand and go clear through in a relatively straight line.
140 grain Nosler Partitions and 130-140 grain Barnes bullet from a 6.5 C are JUST FINE, and so are the PPU 156 grain round nose bullets, but the fragmenting high accuracy bullets commonly used in the cartridge are not what they should be for elk hunting.
Sure they kill elk, -----and so will a 22 LR, but not as efficiently as we would like.
A perfect shot, broadside, that doesn't hit the leg or shoulder bone or shoulder blade is not a good test of any elk bullet. A 9MM pistol will kill elk with that shot.
Outfitters want to know that if you do hit bone, (accidentally of not,) that your round will go clear through in a fairly straight line.
The 6.5 Creedmoor is fine for elk hunting if the hunter loads an elk bullet in it.
I have personally seen more failures on elk with stupid fragmenting target bullets from 7MM Mags and 300 Mags than all the other cartridges. Many of the owners of those guns are falling for the lie that a tighter group on paper is an indicator of a quality elk bullet.
NOTHING could be farther from the truth!
2 MOA with good expansion and full penetration is WAAAAAY better than 1/10 MOA with a bullet that breaks up. I hear and read things all the time about bullet weights for elk and most are from men that don't really understand and have not killed very many elk. If I am asked what weight bullet a hunter should use I always say "80 grains or more.......when the bullet STOPS!"
A 180 grain bullet that weight 40 grains when it stops is not a good elk bullet. But a 100 grain bullet that weights 80 when it stops is an elk bullet.
Remember, it's the bullet hole that kills, not the bullet. You want a hole that goes clear through, and you want a hole through any bone it hits.
7MM Mags and 300 mags are very good calibers for elk hunting, but no cartridge case kills game. Bullet holes kill, and bad bullets many times don't make deep holes and straight holes.
There are WAY too many bullets made today with labels on them, or advertisements about them that say they are for hunting game over 400 pounds when in fact they are not. Many are target/varmint bullets with an outright lie printed on the label.
Keep in mind that a big cow can be 700 pounds and a big bull can go up to 900 pounds.
If you have a 6.5 C with deer bullet you'll love it on deer. But elk are bigger. LOTS bigger. They die fine when shot with smaller rifles, as long as you get good expansion and full penetration.