Railgun Kill
From Halopedia, the Halo wiki
Railgun Kill is a medal awarded in Halo 4 multiplayer and Spartan Ops for killing an enemy with the Railgun. The player will also be awarded the medal if an AI passenger in a vehicle they are driving kills an enemy with the railgun. It is represented by a red lightning bolt passing through four white rings on a blue circle atop a grey four-pointed star.