Railgun

The Railgun is one of the strongest weapons in use in the Warhammer 40,000 universe and it is a Tau heavy weapon mounted on several Tau Fire Caste units including the Hammerhead Gunships and the XV88 Broadside Battlesuit. A Rail Gun uses a series of electromagnets to accelerate its metallic ammunition to speeds close to a significant fraction of the speed of light. The resultingly high levels of kinetic force are then capable of destroying even heavily armoured vehicles. The size of the weapon determines the size of its power source and thus the kinetic force imparted to the ammunition. A Railgun can be used in conjunction with a Marker Light to make it even more accurate. Railguns also exist in a smaller and far less powerful rifle version (the Rail Rifle) used by Tau Pathfinder Teams and Sniper Drone Teams but it is not as reliable as the vehicle-mounted version as it can sometimes kill the user (much like the Imperial Plasma Gun). However, as of the Tau Empire's Third Sphere Expansion, advancements in the technology have eliminated the possibility of a misfire.

Railgun Mechanics
The Railgun functions through the manipulation of electromagnetic forces. A magnetically responsive projectile in an electrically conductive sheath is inserted between two conductive rails attached to a power source, making a complete circuit. As the current moves through the circuit it generates a magnetic force that pushes outward, forcing the projectile down the rails. The further the projectile goes out, the greater the current that goes through and the stronger the magnetic force becomes, causing the projectile to accelerate along the full length of the rails, achieving a tremendous velocity by the time it passes the end of the rails and breaks the circuit. After this, another round can be readied as soon as enough charge can be built back up for the next shot.

To achieve a charge necessary to fire a shell at combat velocity typically requires a bulky array of capacitors attached to a powerful generator, which tends to limit the use of the battlefield Railgun to dedicated vehicles or Tau Battlesuits specifically designed to accommodate them. The Rail Rifle gets around this issue with several pre-charged disposable capacitors which are replaced after every shot. While a Railgun is conceptually simple, manufacturing and fielding one presents several engineering challenges, requiring highly conductive materials with strong resistance to thermal expansion, efficient cooling systems, very precise machining processes, and methods of quickly servicing and re-machining the rails in the field to keep their interior surfaces in pristine condition as even minor wear could have a significantly deleterious affect on the weapon's performance.

Heavy Railguns
Heavy Railguns are larger versions of the standard Railguns and are only mounted on the Manta and A-X-10 variant of the Tiger Shark aircraft. Heavy Railguns fire larger rounds at higher speeds, and the projectiles are fin-stabilised to provide extra lift, and therefore extra range, whilst in an atmosphere. The Heavy Railgun's submunition shell is equipped with a drone processor that is programmed to direct the shell accurately towards its target. This capability is mostly used during void combat to find weak points in the armour of enemy starships as standard submunition rounds lack the necessary penetrating power. They are also useful in ground combat, however, as the shell can saturate a extremely large area with its submunitions and deny cover more effectively to enemy forces due to being fired from the air.