From Halopedia, the Halo wiki
Be the Bullet! is a spree medal in Halo: Reach Multiplayer, Campaign and Firefight. It is awarded for killing fifteen opponents with a Sniper Rifle without dying.
Note that you don't have to get fifteen sniper headshots in a row, just fifteen kills with a sniper rifle. Melees with the sniper rifle do not count toward the medal.
It is represented by a silver six-pointed star with purple crosshairs in the middle.
A player earning the medal in a Firefight game on Outpost.
Halo: The Master Chief Collection version.