Re: hunting caliber AR upper
Hornady is coming out with Grendel ammo, wolf makes it (although underpowered)
I would probably lean to a 6.8 with a SPC II chamber with a slow twist barrel so I could run the SSA combat loads (85 grain TSX at about 3000 fps from a 16" barrel) adding another 4" to the barrel will only help.
I like the 7.62x39, it is a good round, but is clearly inferior to the grendel or 6.8.
If you reload, get the grendel, buy wolf grendel ammo to start with and reload the brass.
You can't go wrong with either of the 2.
To me, the .30 AR is a non-starter, difficult to find ammo, stubby little brick like bullets that lose velocity quickly. Available in any bullet weight you want as long as it is 125 grains. As long as your shots are close, it will work just fine, but if you have to shoot across the corn field, you better learn the art of in-direct fire (probably helps to have been a mortarman). There is absolutely no room for error left on the bolt face (much like the grendel)