Book: Vex Arsenal.
From a tactical perspective, the Slap Rifle is a Vex directed-energy weapon that fills their analog of the light infantry role. From an engineering perspective it's something much more interesting: a terminal. The Slap Rifle receives a bolt of Solar energy from somewhere (or somewhen) else and it points it at a target.
The terminal's flexibility is impressive. In non-combat conditions, the Slap Rifle seems like it might serve as a viable field transmitter, construction tool, navigational beacon, network repeater, or any of a number of other utility functions.