Ok well got lots of good responses and that's why I asked the question. So from my personal experience as a hunter and active duty military here is my opinion. Bullet type and shot placement can make this round deadly to any game all the way up to an elephant, however I wouldn't try it myself. Why? Because it would be inhumane, just as it is for us to use .556 in warfare. For those who have never served in the military while we are at war we have to abide by LOAC aka the Law of Armed Conflict. I'm not going to spew the whole thing but do recommend you look it up and read it. However according to the LOAC military cannot use hollow tip rounds unless warranted by Washington. We are to use FMJ and are restricted to how and when we can shoot at someone. So this brings me to shot placement if you are a great shot regardless of FMJ or HP you can make a kill shot. However I have been hunting quite a few times and I've never run across a deer with an AK47 or a bear with a SKS. (Funny image in my head) SO rushed shots should never be an issue with hunting as they are when fighting combat. Bullet type is a different story though. You can be an ok shot and use a round that will expand on impact creating massive internal trauma and cause blood loss that a FMJ couldn't provide.
My thoughts =
1. Sniper with a .556 you can shoot up to a large Black Bear
2. Great shot with the .556 you can take a large White Tail or Hog as long as you have traumatic ammo
3. Ok shot with any rifle I think a coyote would be ok to hunt with a .556
4. Poor shot would probably say prairie dogs, because as long as you can hit them more than likely they will die
I'm not trying to step on anyones toes here, and I know everyone above that wrote in this forum is a "Sniper" with any rifle they grab, but for most common hunters out there I would say smaller game than Deer or Hog would be the way to go with a .556. No matter human or animal when shooting something I do not want it to suffer or even worse run away and become untrackable. "One shot, one kill" should be every hunters and soldiers motto as they get ready to pull the trigger.