It seems the longest kills have been in the 600-700 yard range. Nothing seems to be written in stone because obviously that distance isn't all that impressive compared to the heavier calibers and what they can do. But it certainly would be possible to kill someone at much longer distances despite what so many naysayers think. I looked around the net and saw literally hundreds of really stupid comments on the subject talking about how a .223 round has no chance of hitting anything at 1000 yards. That's pretty stupid. They will not match the accuracy of heavier bullets at that distance but they certainly can't be shot fairly accurate at 1000 yards. Check out this video of a Savage 12 shooting 1000 yards in very windy conditions.
https://www.youtube.com/watch?v=2H-l5fq8oxs
Obviously it took a while to get the wind figured out and probably a good while to figure out the drop too. But it's possible to hit a man at 1000 yards. That does not mean it has been done. I have no evidence of that. I do have evidence of varmints being hit at that distance.
It should be pointed out that a Savage 12 is a long range shooter. A military issued AR-15 or M4 isn't. It matters. I've shot AR-10's with .308 rounds at 500 yards and they didn't come close to matching what my .223 Savage 12 was doing. It shot the AR enough to know it just wasn't ever going to be as accurate. They certainly do make some AR's that are accurate but the military doesn't give those out in 5.56 form AFAIK. If they want a long range shooter they give him something more likely to result in a kill if he hits his target at long range. It's likely a crap shoot whether a .223 will even kill a person if you did shoot them at 1000 yards. The round is going to be very low on energy by that point. Ground hogs may not be a problem but hitting a man in a vital spot is another story. AFAIK it has never happened. It probably isn't even tried because there are better choices available for a shot like that.
Again I couldn't find any listing of long range shots with a .223/5.56. I heard people talking about 600 yard kills. There is nothing specific even for that. I know my own .223 is capable of hitting targets at 500 yards consistently. That's the farthest distance I have a place to shoot. I could be shooting 600 yards if I was back at my old club I guess. There's another club locally that has a 600 yard range but apparently you have to be a member for a couple of years to even get to shoot there. I'll wait until I can get back home to my old club.
Good luck finding data on .223 kills and be sure to let us know if you find anything solid.