For the most part the alternate cartridges for the AR just move the ballistic performance we can get from one end of the envelope to the other. It's no different than building a motor, you can go for low end torque or high end horsepower.
Given the constraints of a finite overall length and maximum bolt face diameter, if you want more power out of a shorter barrel, you increase the case diameter to the maximum possible and then add as much bullet weight as possible. Is .300 BO a better answer than 5.56 doing that, yes. The DOD won't entertain the idea for logistical reasons, and even tho the 6.8 was developed in house, it isn't used for the same reasons.
That means the professionals don't consider the incremental increase in single round bullet weight and it's delivered power transfer to be worth the bother. Why? The battle concept of the M16 is to shoot ANOTHER round if needed. Same for those using SBR's in shipboarding or infra urban operations. If one isn't enough, we've got minimum 300 more.
It's the civilian world looking for the maximum power transfer who keeps bringing it up. And that is based on wanting to either have so much power they literally force the target thru walls or want them field dressed on the spot. It's a focus on what they have seen in movies and on TV - much of which is simple fantasy.
Real high explosives aren't large flaming fireballs towering into the sky, a real hit on a live target isn't going to knock them down anchored on the spot Dead Right There. Trying to get that out of a bullet with a finite weight limit and diameter constraints means nobody is going to come up with The Magic Bullet they expect. Not even close.
And when you do push the limit to the maximum, what you get are the ".500's" like the Bushmaster or SOCOM. At that point you are forced into using the AR10 bolt with undersized rebated rim, and the costs of ammo are exponentially even higher. Basically you took a stock motor to it's limits with bore and stroke. It's expensive - you do get the performance up - but it still leaves something to be desired because the goal is unrealistic to begin with.
You can't put a 406 in a Miata and expect that it will do. Nope, next you will want a 500 and supercharge that. What most gunners fail to accept is that the rest of the platform has to be modified to accept it or bad things happen to parts so highly overloaded their mean rate of failure goes straight into the dumpster.
Cartridge wars are a complete waste of time as most of the participants are too narrowly focused on the incremental differences to see the big picture. If you want power, may I suggest .50BMG. Otherwise accept the fact that our small lightweight poodle shooters do just fine as the weapons they were intended. What the owner needs to do is step up to how they get applied correctly rather than spin their wheels in choking clouds of smoke demonstrating how studly they are. Real firearms use is not a horsepower race for bragging rights.