From Halopedia, the Halo wiki
This article is about the medal. For achievement, see Sniper Kill (achievement).
The Sniper Kill medal (Sniper Rifle Kill in Halo 4) is a medal awarded in campaign, multiplayer, Firefight and Spartan Ops. In Halo 3 and Halo: Reach it is awarded for killing an opponent with a headshot using the Sniper Rifle or Beam Rifle. In Halo 2, Halo 4, and the Halo 3 Beta it is also awarded for non-headshot kills. In Halo 4 and Halo 5: Guardians the medal is only awarded for kills with the Sniper Rifle as the Beam Rifle and Binary Rifle award their own medals.
In Halo 2 through Halo: Reach it is represented by purple crosshairs on a six-pointed gold star. In Halo 4 it is represented by white crosshairs on a blue circle on top of a grey four-pointed star.
After you kill an opponent with a sniper weapon, a message appears on your screen that says: "You sniped (player)".