Traildigger....Plug away on the 6.8SPC, it's not that bad of a round, as long as you keep it within it's limits it will perform very well on deer sized game, hell, it will do on bears as long as you are mindful of it's abilities and use a good bullet.
For coyotes we found the 110 V-Max blows them all to hell. The Speer 90gr TNT went through without much expansion and several dogs were lost due to the bullet zipping right through. There are some newer bullets on the market from Hornady and Barnes that look promising but I simply did not have time to use them before I sold the 6.8's. 400 yards is doable but accuracy is the key and we never found the 6.8 to be great at those distances so we stayed with bolt guns for that purpose.
I now shoot a 6x45 and it has a hell of a following but again, it's limitations keep it from being a stellar round but it will kill deer all day under 300 yards just as the 6.8 will do.
Good luck with it, it's a nice round for the AR15 and if you put a brake on it, it will recoil like a 22LR. Just a bunch louder