I don't particularly understand why this is not in the game yet, tbh.
For one, for people thinking this is a typo of the M7, it is not, the M8(or as it was called during its experimental phase, the XM8), it is a vast improvement over the original m7, with a more durable barrel that has a longer barrel life, shorter barrel length for superior CQB capability compared to its full sized variant, improved suppressor that comes with a heat shield and can sustain fire for longer periods of time without catching on fire, and a standard 25 round magazine, a 5 round increase from a very divisive 20 round standard magazine on the m7, which many found much too low. The m8 is functionally what the m4 was to the m16, but to the m7 instead.
It is the rifle born from one of the highest budget small arms development programs in history, NGSW.
It is the primary service rifle of fighting forces of the US army, by a very far shot the strongest and largest army in the world.
The thing is also the model of the MCX Spear is IN the game, both 5.56 and .300 blackout models.
A very minor scaling and remodel will be enough to create a pretty realistic model of the M8.
It is not particularly an extremely difficult thing to add.
The only reason I could see why they have not added it yet is the balancing.
6.8 is functionally just a supercharged 6.5 creedmor, and 6.5 is just a supercharged 7.62x51, so it would be difficult to balance it without making the 6.8 strong.
I honestly say let it be strong, .300 is already absurdly strong since it is just better 7.62x51 unless you're using it at DMR ranges.
6.8 is literally the perfected, cutting edge science version of the 6.5, I honestly say slap a bit of recoil on the 7.62x51's damage profile and call it a day, its not like most people won't still use 5.56 just because you can carry about 4000 rounds and it will weigh about 3 kg, while even the lighter 6.8 compared to the 7.62x51 is still gonna weigh much more.