The LRG Rail Gun is a combined anti-vehicle/anti-structure weapons system mounted on the SP42 Cobra self-propelled artillery vehicle, designed for engaging enemy armored vehicles and structures, using a pair of combining Railguns. The individual railguns are also able to be mounted on a UNSC prefabricated base turret for use as a static defensive measure.
The LRG Rail Gun is one of a few weapons of its kind in service. Most known UNSC weapons that use electromagnetism to fire solid projectiles, such as the MAC or the M68 Gauss Cannon, are coilguns which operate using completely different principles. Rail Gun operation often features a linear acceleration motor. This motor generates a strong electric current which travels from the motor, to the "positive" rail, through the projectile, and through the "negative" rail-completing a circuit. This generates forward motion caused by 'Lorentz Force.' Rail Guns can accelerate projectiles at far greater speeds than Magnetic Accelerator Cannons, also known as Gauss Cannons. However, Rail Guns require far more power to operate and their rails receive great wear and tear from each shot.
The most unique aspect of the LRG Rail Gun was that it featured two separate firing modes, each performing completely separate roles and duties. At its most basic stage, it used two M66 railguns to fire 30mm armor piercing shells, with significant penetration capabilities, designed for engaging Covenant armored vehicles. Each railgun has a slow rate of fire, and requires alternating fire - the energy required to fire both railguns together is well beyond what the Cobra is capable of producing by itself. This alternating also improves the weapon's rate of fire, keeping up a more consistent output, with superior penetration to the 90mm High Velocity Cannon used by the M808B Scorpion MBT. Further, these two railguns can be retracted into the Cobra to allow the deployment of the larger M98 railgun. This stronger weapon fires a 105mm high explosive shell at significant range, filling a more conventional artillery role. This second mode of firing requires more stability than a mobile vehicle could provide, forcing the Cobra to deploy support stands and braces to keep it stable, but allows extreme long range fire, effective against vehicles and enemy structures. Upgrades are available to further improve the artillery cannon's penetration, allowing a single shell to penetrate through a target and hit a second behind it.
- Though the Halo Wars website and strategy guide initially labeled the weapon as a rail gun, there has been debate over why the gun was originally called M66 Gauss Cannon in concept art. Since the game's release, references to the M66 have been deleted on official material. As the two types of weapons operate on completely different principles, it would have been impossible to claim that both names were accurate.
- Halo Wars (First appearance)