Hornady has came out with a new round perfectly suited for the AR platform or bolt guns.
Rumor has it that a certain military agency has adopted it as a replacement cartridge.
Basically it's a 6.5 Grendel necked down to 6MM. Pretty impressive ballistics that will greatly out perform the 5.56 and the .308 that only comes in the heavier plarform AR-10. The 5.56 has had issues with lethality after penetrating a barricade or at ranges past 300 yards or so.
Bullets for reloading range from 55 grain to 108 grain.
More info at https://www.hornady.com/6mmARC#!/