From Halopedia, the Halo wiki
"Sharpshooter" is a spree medal awarded in Halo 3, Halo 3: ODST and Halo: Reach Multiplayer, Campaign and Firefight. In Halo: 3 and Halo 3: ODST it is awarded for killing ten opponents in a row with the Sniper Rifle or Type-50 particle beam rifle (together or not) without dying. In Halo: Reach the medal does not require kills to be consecutive, just that the player kill ten enemies with a sniper weapon without dying.
It is represented by a bronze five-pointed star with green crosshairs.