Okay, here is what Proof Research shows they make for the AR-15:
View attachment 141227
You are asking for advice. My advice is "Hogs are just as difficult/easy to kill as deer. If you wouldn't use it on a deer, don't use it on a hog."
With that being said, and with the choices Proof offers, your options are:
6.5 Grendel
6.8 SPC
.300 BLK
Any of those are good!
If you go by over-the-counter ammo availability and variety:
1) .300 BLK (Blackout)
2) 6.5 Grendel
3) 6.8 SPC
If you go by 'flattest shooting/most range':
1) 6.5 Grendel
2) 6.8 SPC
3) .300 BLK
If you go by "energy delivered at 100 yards":
1) 6.5 Grendel/6.8 SPC = virtual tie
3) .300 Blackout (about 400 fpe LESS than the other two @ 100 yards)
If you go by LEAST to MOST recoil:
1) .300 Blackout (softest kick)
2) 6.8 SPC
3) 6.5 Grendel (most kick)
...but if we are honest here, you'd be hard pressed to "feel" a difference between the three of these out of an AR-15 as none of them are 'hard-kicking' in any sense of the word.
Here is a good article for you to read, if you haven't already made up your mind to go with one of the .22 caliber rounds:
https://www.chuckhawks.com/ar-15_deer_cartridges.html
and/or
https://www.chuckhawks.com/compared_6-5_6-8_300_350.html