From Halopedia, the Halo wiki
Shotgun Spree 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 Shotgun without dying. In Halo: Reach the medal does not require kills to be consecutive, just that the player kill ten enemies with the shotgun without dying.
Melees with the shotgun do not count toward the medal.
It is represented by a bronze five-pointed star with three green shotgun shells. This medal can be easily obtained through many maps, but is easiest to get on common camping spots or in objective-based games where the focus is drawn away from killing. Especially on maps such as Epitaph, Foundry, Guardian, and Orbital.