My longest was an antelope at 970 yards. The wind is the most important factor, the rest of it is just math, a very good rifle, good optics, and practice.
At a 1000 yds a 2 mph cross wind can move a bullet a foot, and it gets a lot worse the further you go, and how do you knw what the wind is doing between you and the target. You just need the wind gods on your side.
That's pretty darn good too. Care to share some details: rifle, caliber, load etc.?
Was it a one shot kill?