The military has been playing with the 6.8 SPC from Remington since I first fired an Australian F89 Minimi in that chambering in early 2000, its primary consideration being the arrangement of suitable links for feeding in full-auto SAWs and in the then-popular H&K XM-8.
Of late, SOCOM has given the .300, often silenced, a good workout, especially with night scopes, and has reportedly made in excess of 1000 kills in Afghanistan and Syria with it. It's main disadvantage is its ability to have the .300-diameter bulleted cartridge chamber in a regular 5.56mm M4, pushing the bullet back into the cartridge case and then firing, the .30 bullet NOT going down the smaller diameter barrel. Special Forces and other SOCOM users are well-enough trained that this is not usually a problem for them, but general service-wide issue would be too likely to cause problems. The 7.65 Wilson is an improvement that avoids this, but though it too is being tried, it's still a test case.
And then there is the .350 Winchester Legend. Too new to tell, but offering promise, we'll see. I want one with a gas-operated piston system in an AR-18 upper.
I've had good success with the 300. Probably the best subsonic i’ve ever worked with. Cycles well using a pistol gas tube. Have my glass zeroed for subsonic then dial it to the stop for my supersonic. 220’grain for sub and those long black tipped Barnes 110 for super.