Tau Cannon (D20 Modern Equipment)
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Tau Cannon []
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 Tau Cannon
A prototype for the assault rifle of the future, the Tau Cannon is an experimental weapon developed by scientists at Black Mesa. Its advanced technology allows it to use depleted uranium as fuel. Furthermore, you do not have to worry about reloading the gun – it taps directly into your ammo reserve. The depleted uranium ammunition is shared by the gluon gun. The tau cannon is also known by many as the "gauss gun", as many of the Half-Life game files refer to the weapon as the "gauss", though this is simply a relic of Half-Life’s alpha development stage.
The tau cannon is a type of experimental weapon that fires beams of Tau particles. The weapon has two different fire modes. The first shoots tau particles rapidly, visible as yellow beams in the air; each beam damages a target with its negative charge and its bullet-like kinetic energy. Though very inaccurate, these rapid discharges can quickly gun down unarmored targets. The alternate fire mode charges up the weapon to fire more powerful white-hot beams; these can shoot through multiple targets or penetrate walls, causing a devastating explosion-like blast of particles on the far side. The tau particle beams can reflect off walls if they are hit indirectly, and in fact can be reflected several times. This aspect can be useful as it can be used to hit targets when there is no clear shot, but as with bullet ricochets there is also a danger of friendly fire or self-harm. Tau cannons are compact and light enough to count as small arms, despite the fact that a blast from a charged-up tau cannon can penetrate or destroy armored vehicles.
The tau is the most massive member of the lepton family of elementary particles; like electrons, tau particles are negatively charged, but tau particles have more than three thousand times as much mass. This would explain the tau cannon’s powerful penetrating power, as well as its significant recoil. Though the early tau cannon prototypes used normal household batteries as an initial power source for the particle accelerator, a micro-fission reactor generated the deadly particle beam. This advanced nuclear device, developed at Black Mesa, was capable of using inexpensive depleted uranium as fuel rather than an enriched isotope. (The same technology was put to use in the experimental gluon gun that was developed during the same period.) When the tau cannon alternate fire mode was used the reactor would be fed more depleted uranium fuel without being fired, and the energy would be stored in a set of spinning capacitors. Although these prototype capacitors could store a vast amount of energy they could not hold a charge for long, and if the tau cannon’s capacitors were fully charged-up the weapon would have to be fired within ten seconds to prevent a catastrophic discharge that could kill the user of the weapon.
The vehicle-mounted tau cannon used during the time of the rebellion against the Combine is a more advanced model. It uses an alternative fuel source that gives it seemingly unlimited ammunition, and the capacitors can hold a full charge indefinitely. The weapon is devastating against Combine soldiers and alien life-forms, but useless against Combine vehicles as their advanced armour is designed to reflect energy weapons.
See  for background information.