My friend George shot a doe antelope with a 6.5 Creedmoor using a 140 grain bullet. I watched the hit through a 6 to 18 scope on 18 power. I saw the bullet hit just behind the foreleg. We lost the doe because it was able to still ran. I know that the 6.5 X 55 is used in Europe for moose and red deer (or stag) but I wouldn't use it. I also know that several of the gentlemen writing on this blog lost elk last fall because the 6.5 CM didn't kill effectively. I'd say that if you're going to hunt with this rifle, stay with the .308 and use the Nosler ballistic tip in 180 grain, or the Sierra Gameking in the same weight. If you barrel with a 24 inch barrel and shoot a 175 to 180 grain bullet, you can get high BC, high sectional density and a velocity of around 2600-2650fps which will give you a good trajectory, higher impact, a larger wound channel, and much better penetration on big game. Also, keep in mind that the bullets for the 6.5 are mostly target bullets with their limitations where hunting is concerned, while most of the .308 bullets are designed with big game and terminal performance in mind. I'd stay with the .308.
I killed a truckload of Antelope and Mule Deer with a 25-06 & 25-06 & my Wife did quite well with a 6.5x55, good thing I did not know I was undergunned.
My Wife now also has a 6.5CM, she uses it interchangeably, the Swede shoots the 140 grain Nos. Partition 2700fps, the CM runs just over with the same bullet. She has taken 2 Bull Elk, one with each rifle, same results, the 140 NP is a great bullet & to match the Sectional Density with a 30 cal, it would have to be 190 grain+.
Both went down fast, neither were long range, I don't think it would be a good choice past 300 for Elk, but I would rather go with a 300 for that anyway.
I hunt with alot of different cartridges because I am a rifle looney but have used the 308 for MANY animals here in AR, bunches of Hogs & Whitetails, meanwhile my Wife has taken 40 or so Whitetails with her 6.5's, we compare notes & her results and mine are pretty much the same. At "normal" ranges & with good shot placement with proper bullets & it's time for the shinning knives.
A good thing to watch is a big series of videos by a guy that goes by "Zen Archery", you can watch him take dozens of Hogs with various 6.5 bullets out of an AR in CM. & yes the big Hogs at least are a little tougher than Deer.
FOR HUNTING, pretty simple to me , I am very comfortable with a 308 or CM/260/Swede for medium game up to 3-400 yds, some think further & that's OK & I want a larger cartridge of either caliber at long ranges & would personally prefer a 300, realizing it is not mandatory but again preferred.
FOR target work, the CM shoots a hair flatter & bucks the wind better with less recoil with the best loads for both.
This deal of thinking a 6.5 is not enough for Deer/Hogs/Black Bear at medium range just cracks me up & I have to wonder about experience at that point.
As far as bullet choices, both calibers now have plenty of choices for hunting & target work!