Testing and DesignEdit
During it's testing and design phase, it was known as the "X555 Electromagnetic Launcher Advanced Technologies Testbed".
ONI Railgun - A variant of the M555 Elctromagnetic Launcher, where the slugs produce lingering area-of-effect damage.
The railguin can be found in tripod form. Like otherguns, it can be removed from its mount. Picking up the railgun results in a lack of mobility. The gun behaves like the M68 Asynchronous Linear-Induction Motor in previous Halo titles.