With thermal, the ability to take multiple shots quickly makes the AR a very attractive weapon. I've always liked bolt-actions and used a 243 Ruger Predator with my Apex for about 6 months. Killed a good number of coyotes with it and made some long shots, but I realized I was not taking the fullest advantage of the thermal's ability with the bolt gun.
So, I put together a 6x45 upper for one of my AR's. There are other options of course, standard .223, 22 Nosler, now the 224 Valkyrie, 243 LBC, 6x6.8, etc., etc., and so forth.
I really like the 6x45. I've determined to call it the 6mm Parvo from here on. If you reload, it is a no-brainer with plenty of .223 brass and dies to make it happen in one pass. Losing brass while hunting is a non-issue with the 6mm Parvo.
I'm shooting 55 gr. Nosler Ballistic Tips at just under 3,400 fps (at sea level). The upper also shoots 55 Varmageddon's and 58 V-max very well plus 70 gr. Ballistic tips. I just like the speed of the somewhat tougher 55 BT's in this upper (20" barrel). No, it isn't pushing them at 3,900 fps like the 243 does easily, but it does whack coyotes plenty hard.
The 6mm Parvo has been a very solid performer for me, and while I like several of the other new AR caliber's I'm just not sure that I gain enough to make it worthwhile. If you don't reload of course then all this goes out the window and you have to go with something in a factory loading like the Grendel.