Not too many years ago several people who questioned the shot by Dixon, got a surveyor, a ballistic specialist from the US Army and a bunch of competition 1000 yard BP shooters and did a study on the tale. The conclusion was ....yes it was possible and probably happened. Pretty well reported and covered in an edition of True West magazine.
Sorry I wasn't too clear, Jim P is right. The shot was from a place called Adobe Walls and the Indian Chief was on horseback on a bluff overlooking the fortification.