I have killed deer, antelope and domestic sheep with the 6.8SPC and the 7.62X39, and I have killed deer and antelope with the 6.5 Grendel.

Bullets that I have used are as follows:
6.8SPC.
100 grain Remingtons. -- 27 kills. (Excellent results, super fast drops and all exited but one. Remington screwed us and no longer sells them however. Accuracy is about 1.2 MOA, which is niot as good as some others, but just fine for shots out to about 450 yards which is getting out there for a 6.8. None of these cartridges are what I would use past 400 yards on game no matter how accurate they were)
110 Grain Barnes --1 kill. Neck shot at a deer. Perfect hit and instant kill, but not a real test of the bullet because a 22 would have dropped it as fast with an identical hit.
110 grain Sierra. --2 kills. Both hits to the chest, one hit a shoulder bone. Both bullet were on the ribs or skin on the off sides, but came apart and left only empty jackets, but both deer, one mid size buck and one large doe dropped instantly.
115 grain Remington FMJs -- 2 kills. Sheep. Both chest shot. Both moved about 4 seconds after the shots and fell. Both had exits from obvious tumbling. Exits were elliptical and about the size of an egg.
130 grain Winchester Power Points 3 kills. All 3 were deer, 1 white tail buck, one whitetail doe and on mule deer buck. 2 fell at the shots and one ran about 15 feet after the hit and dropped in some brush. All left good exits including the mule deer buck which was hit through one shoulder bone.

6.5 Grendel
125 grain Nosler Partitions -- 2 kills. One was a bang-flop and the other put the antelope down in about 2 seconds. Both exited. Excellent results.
129 grain Hornady inner-lock--- 3 kills. All antelope does. Hits were obvious and they ran about 25-40 yards and fell. 2 exits and one good mushroom inside the ribs.
129 grain Hornady SSTs --- 3 kills. 1 antelope buck and 2 antelope does. All went about 30 -40 yards after the hits. 2 exits, one on the buck and one doe. One doe hit from the front, but I was not able to recover the bullet.

7.62X39 123 grain Russian SP --- 5 kills antelope and white tails. All were excellent with all dropping within about 2-3 seconds, and all exits
122 grain Russian hollow points 2 kills Both white tail does. Total fragmentation of both bullets and no exits. deer ran about 30 yards after being hit. One stopped and stood for about 10 seconds after the short run and then fell.
154 grain Russian Soft Points 2 kills. Small amount of expansion with exits the size of nickels. Both white tail does. Deer stayed up for about 10 seconds after the hits.
All the kills listed for this cartridge were made with Russian factory ammo but one. That kill was made with a 123 grain Speer bullet, hand-loaded (military bullet pulled and replaced with this bullet) on a large bodied mule deer. The deer fell in about 4 seconds leaving the bullet on the ribs on the off side expanded well, but the core was loose in the jacket when I removed it. That deer hung at 171 pounds.
123 grain FMJ. 5 sheep. All did exactly what I expected. 4 Dropped the sheep in 3-6 seconds and one fell instantly. All exited. Exit wounds often veer off course in the bodies.

Bullets matter more then cartridge cases, but so far what I have seen is the speed that the 7.62X39 dropped game and the speed of the 6.5 Grendel are about the same.

The 6.8 puts them down faster, but is not as flat shooting as the 6.5 G. or not as cheep to shoot as the 7.62X39.

All work very well if you give them bullet that work well.