Railgun Kill
From Halopedia, the Halo wiki
Railgun Kill is a medal awarded in Halo 4 multiplayer and Spartan Ops and in Halo 5: Guardians 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. In Halo 4 it is represented by a red lightning bolt passing through four white rings on a blue circle atop a grey four-pointed star. In Halo 5: Guardians it is represented by the weapon's reticule atop a four-pointed star.
The Railgun Kill medal in Halo 4.