To date I have used the Hornady 143 gr ELD-X and Federal 130 gr Terminal Ascent ammunition on big game in my 6.5 Creedmoor (LH Browning X Bolt All Weather).
The Hdy 143 gr ELD-X was used an a young bull moose at 20 yards with a frontal shot. The mosse dies within 5 yards of where it took the shot. The bullet was not recovered in the mess in the vitals. I found a bit of jacket that swiped the edge of the ribs as it entered the chest cavity beside the trachea/esophagus, but did not penetrate the diaphragm. While effective and a quick kill, and its high impact velocity at approx 2800 fps, I was not impressed as to how frangible this bullet is. This ammo produces 0.454"' groups @ 100 yards, but is slow at 2510 fps out of my 24" Benchmark 5 R barrel. (Ammo rated for 2700 fps - finding this ammo consistently slower than rated in all rifles/calibers tried to date) I have chosen not to hunt with this bullet moving forward, although happy to use remaining ammo on hand for target practice, and reloading for further target practice.
The Federal 130 gr TA has been used on red stag, fallow buck, 2 arapawa rams and caribou, at ranges from 43 (caribou) to 296 yards (red stag).
All were one shot kills, with complete penetration. And 3 were DRT, while the red stag fell within 5 yards of where it took the shot, and the fallow buck jumped at the shot, and fell 15 yards downhill of where it took the shot. (The two arapawa rams were the first two the guide had ever seen DRT, as they usually run 100-400 yards despite being dead on the hoof). This ammo produces 0.291" groups @ 100 yards art 2855 fps in my barrel. I also have a Winchester Model 1885 High Wall in 6.5 CM and this ammo produces 0.599" groups @ 100 yards at 2911 fps out of its 28" barrel. As a note, red stag and caribou weigh approx 400-450 on the hoof, which is a little larger than mule deer - although I have taken two mule deer bucks over the years that were approx 400 pounds on the hoof! And I wouldn't hesitate to use this ammo on black bears, as this is the ammunition my wife is using in her LH Browning X Bolt Hunter in 6.5 CM for her bear hunting now. Her factory 22" barrel shoots this ammo into 0.420" groups @ 100 yards at 2755 fps. I am very impressed with this bullet and ammo to date; performance wise for accuracy and on-game performance!
While I have not yet taken game with the Nosler 140 gr AccuBond ammo in the 6.5 CM (it is my favourite hunting bullet over the past 20+ years on big game ranging from pronghorn to bison), and produces 0.248" groups @ 100 yards in the High Wall at 2826 fps. I wouldn't hesitate to use this for big game, within reasonable distances where the retained velocity and energy is proper for the animal (1000 ft. lbs for deer/bear, 1500 ft lbs for moose, and 2000 ft lbs for elk/bison).
While the Winchester 129 gr Power Point ammo is not as accurate in my rifles, 1.251" and 1.165" groups @ 100 yards at 2810 and 2941 fps respectively from the Browning and Winchester rifles. While I have not taken game with these from the 6.5 CM, I have used Power Point bullets in other rifles/cartridges over the years and have always been happy with the performance of this cup and core bullet. Again, I wouldn't hesitate to use it on big game, given the same parameters as stated above with the AccuBond.
While not the 6.5 CM, I have used 140 gr Nosler Partitions and the older Barnes X bullets on stones sheep, moose, whitetail, and a cow elk out of the 6.5x55 Swedish Mauser over the years. I wouldn't hesitate to use the Partition in the Creedmoor too.
While I have not used Sellier & Bellot ammo in the 6.5 CM, I did use their ammo in the PH's rifle (300 Win Mag) in Africa last year and was impressed by the performance of the SPCE (controlled expansion) bullet on the game ranging from springbok to kudu, at distances from 77 to 235 yards. All were one shot kills, with only 3 of 7 bullets recovered. Avg weight retention 48.7% and 2 times expansion. I wouldn't hesitate to use this ammo in the 6.5 CM on big game.
As stated above, I do prefer a larger caliber and/or cartridge for elk...but if within 200 yards on an unwary elk, I would use my 6.5 CM, and stay off the shoulder. For my wife or another less experienced hunter, I would likely limit them to 100 yards on unwary elk. On a rut crazed bull or wary animal, I would pass on the elk with the 6.5 CM. But this is my experience, having taken more than 20 elk over the years.
In the end, find the bullet , or ammo that performs best in your rifle and go forth with confidence, that when you place these bullets in the vitals of a big game animal, with sufficient retained energy for proper expansion and penetration, produces a quick kill, that you will need to put your knife to work!
PS, if you are new to hunting, check your provincial hunting regs, as in most places fmj bullets are not legal for hunting