"One of their light walkers carried a weapon of lethal effect. It fired a form of ultra-high velocity projectile. I saw one of our tanks after having been hit by it. There was a small hole punched in either flank - one the projectile's entry point, the other its exit. The tiny munition had passed through the vehicle with such speed that everything within the hull not welded down had been sucked out through the exit hole. Including the crew. We never identified their bodies, for all that remained of them was a red stain upon the ground extending some twenty metres from the wreck."
- — Major Kane, Mordian 607th Heavy Armoured Regiment
A Heavy Rail Rifle is a large Tau ballistic Rail Weapon that makes use of electromagnetically-induced linear acceleration to fire a projectile at hypersonic speeds. A Heavy Rail Rifle is a potent weapon that makes use of the same technologies as the larger Railgun. However, the Heavy Rail Rifle is intended for use on a slightly smaller scale so that it can be mounted on Tau Battlesuits. Whilst the weapon’s range and damage is not as great as its larger cousin, it is still an impressive anti-tank weapon, renowned for its deadliness to enemy armour at extreme ranges. The cylindrical projectile it fires can hammer straight through Power Armour and even redoubtable Tactical Dreadnought Armour with ease, passing through at such incredible velocity that the sudden burst of pressure followed by violent decompression reduces all living matter inside to a tangle of semi-liquid mush. Even a Space Marine’s enhanced physiology is no match for such pinpoint devastation. The Heavy Rail Rifle is roundly hated by the Adeptus Astartes Chapters that have encountered it, for not only does it kill their battle-brothers upon impact, but it destroys or renders inoperable the Progenoid Glands that allow their Apothecaries to create more Space Marines from the fallen. In this way, it not only kills the Adeptus Astartes of the present, but also kills the warriors of the future. XV88 Broadside Battlesuits are the only Tau unit that are armed with Heavy Rail Rifles, and make use of them as a twin-linked primary weapons system.
A Heavy Rail Rifle uses a series of powerful electromagnets and super-conductive electrodes to accelerate small, solid and conductive projectiles to hypersonic speeds. The resulting high levels of kinetic force imparted to the weapon's projectile then allows it to devastate and destroy all but the most heavily armoured vehicles on impact, even at exteme range. Because the projectile is capable of achieving a velocity of anywhere between six and ten times the speed of sound, it generates a tremendous force that is often far more than would be produced by an explosive charge of the same size. As such, the speed of the projectile is enough to punch through even the thickest of enemy armour. The size of the weapon determines the size of its power source and thus the kinetic force imparted to the type of projectile used as ammunition.
Heavy Rail Rifles usually fire single, solid projectiles and rely on the projectile's speed to penetrate armour, even at long ranges. Because the projectiles are so small, large numbers can be carried by the weapon's operator at a time. Such is the power of an XV88 Broadside Battlesuit's Heavy Rail Rifle shot that it can penetrate the thickest plasteel bunkers, often doing so with enough force to punch cleanly through both sides of the structure. However, when used against infantry, a Heavy Rail Rifle can have a devastating effect where the round will simply vaporise the target. Those near the path of the projectile will also find the breath sucked out of their lungs, such is its speed. A Heavy Rail Rifle makes an iconic "whip-crack" sound when it fires, which is created when the Heavy Rail Rifle's round breaks the sound barrier. Many enemy soldiers have learned to dread it as its distinct noise could only be heard after the round had already hit home.
The basic physical principles behind Rail Weapon technology is well-known to the Adeptus Mechanicus, but the Imperium has never been able to utilise it in a tactical form due to the myriad issues inherent in the operation of such weapons. The limitations of such technology include the staggering amount of power required to propel the projectile along the rail, and the dissipation of the heat generated by the process. The Tau appear to have implemented solutions to these problems, and many Hereteks amongst the Adeptus Mechanicus would very much like to understand how.
Rail Weapon Mechanics
A Heavy Rail Rifle functions through the manipulation of electromagnetic force. 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 an electromagnetic force that pushes outward, forcing the projectile down the rails. The further the projectile goes, the greater the current generated and the stronger the electromagnetic 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 the charge necessary to fire a shell at combat velocity typically requires a bulky array of capacitors attached to a powerful generator, which an XV88 Battlesuit is able to provide through its Particle Accelerator power core. While a Heavy Rail Rifle 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 can have a significantly deleterious effect on the weapon's performance.
- Codex: Tau (3rd Edition), pg. 17
- Codex: Tau Empire (4th Edition), pg. 30
- Codex: Tau Empire (6th Edition), pp. 42, 66
- Deathwatch: Mark of the Xenos (RPG), pg. 6
- Imperial Armour Volume Three - The Taros Campaign, pp. 68, 175, 198, 208, 284
- Rogue Trader: Tau Character Guide (RPG), pg. 38
- Warhammer 40,000: Dataslate - Firebase Support Cadre (Digital Edition)