Railgun

A railgun, alternatively spelled rail gun, is a type of electromagnetic projectile weapon. While often confused with Magnetic Accelerator Cannons, rather than accelerating their projectiles through the use of magnetic coils, railguns, as their name suggests, utilize magnetic rails to perform this task. More precisely, a railgun accelerates a conductive projectile along a pair of metal rails, using two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts with the strong magnetic fields generated by the rails and this accelerates the projectile.

Humanity's first tests with railguns took place in the early 21st century. While coilguns are more common in use during the 26th century in form of MACs, the UNSC also uses at least two types of railguns; the Twin Defensive Rail Gun Turrets on UNSC Frigates and the LRG Rail Gun.

LRG Rail Gun
The LRG Rail Gun serves as both the primary and secondary armament of the SP42 Cobra anti-vehicle/anti-fortification system, and is also used as an anti-vehicle weapon on automated defense turrets. The 16 megajoule model of the weapon it used as a potent anti-vehicle weapon, while the 8 megajoule version is used to demolish structures.

Twin Defensive Rail Gun Turrets
Not much is known about this type of weapon, but given their size, it is likely they are used in point-defense role against enemy fighters or in close-range combat with larger vessels.