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Tau Cannon

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"Man, why aren't we using it?!"
Black Mesa Security Guard[src]

The Tau Cannon, also known as the XVL1456,[2] is an experimental energy weapon introduced in the Half-Life chapter Questionable Ethics, on the second floor of the Advanced Biological Research Lab. Due to its strength and versatility, it is considered by some to be the best weapon in the game. If the player chooses not to pick it up in the Research Lab, it can be obtained in the Lambda Reactor Core supply depot at the end of the chapter Lambda Core.


Tau Cannon


A prototype energy-based assault rifle, the Tau Cannon is a weapon developed by scientists at Black Mesa. Its advanced technology allows it to use depleted Uranium-235 as fuel. Furthermore, Gordon does not have to worry about reloading the gun – it taps directly into the ammo reserve. The depleted Uranium-235 ammunition is shared with the Gluon Gun.

The Tau Cannon 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 only moderately per hit, these rapid discharges can quickly gun down unarmored targets. The alt-fire mode charges up the weapon to fire more powerful white-hot beams; these can shoot through multiple targets or walls, causing a devastating explosion blast of particles on the far side, thus gibbing and knocking any target that gets in the beam. In multiplayer this means that it can be safer to jump around in the open than duck behind cover if the opponent is armed with a Tau Cannon. Furthermore, in multiplayer the recoil from the weapon’s secondary attack is greatly increased, and if the gun is fired at the floor, the recoil will kick the player backwards high into the air, propelling him to a higher location. This technique is commonly known as ‘gauss jumping.’

The tau particle beams can reflect off walls if they are at the right angle, 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. The Tau Cannon is compact and light enough to count as small arms, despite the fact that a blast from a charged-up Tau Cannon can penetrate or even obliterate armored vehicles.

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. When the Tau Cannon alternate fire mode was used the reactor would be fed more depleted Uranium-235 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 would cause 50 points of damage for 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, so the player doesn't have to worry about catastrophically discharging the weapon. However, the player can still hurt himself if the beam hits a wall too close to the player. The weapon is devastating against Combine soldiers and alien life-forms, little three to four shots is powerful enough to kill a soldier, and a fully charged can knock a soldier from a distance. However, the Tau Cannon is useless against Combine APCs and Combine Gunships as their advanced armor is designed to reflect energy weapons, unlike in Half-Life where the player can destroy a helicopter, HECU APC and the Tank.

Behind the scenesEdit


A Gauss Gun (or coilgun) is a device that uses electromagnetism to accelerate projectiles, and has some similarity to a railgun. Early during Half-Life’s development Valve intended the weapon to be a hypervelocity projectile weapon (as it is referred to by an unused HEV sound clip, saying "Experimental Hyper-velocity Projectile Weapon Acquired"), but later changed their minds and made it into a particle beam weapon. The weapon uses the same ammunition as the gluon gun, which is obviously an energy weapon, giving further evidence to the weapon not being a true Gauss Gun. The evidence for the weapon’s real name can be found in a game of Half-Life: Deathmatch: in the game console a kill from the weapon is recorded as being a death by ‘tau_cannon.’ When the Opposing Force: Capture the Flag multiplayer game was released the key configuration menu included the option to bind keys to particular weapons; the weapon in question is listed as the Tau Cannon.

Half-Life 2Edit

Besides being mounted on a Scout Car, the Tau Cannon was first intended to be one of the weapons that could be used by player as an identical model, but it was cut out of release version, likely due to its similarity in function to the OSIPR.

An unused sound clip (gettauoff.wav) can be found in the game's files of a citizen talking about detaching a weapon - deduced to be the Tau Cannon due to the name of the file - from the Scout Car, suggesting that at some point in the game's development, the Tau Cannon was to be detached from the car for the player to take with them after leaving the car at Lighthouse Point. Also the "script_tauremoval" Half-Life 2 entity means that a vortigaunt originally should have detached the Tau Cannon from Scout Car. The Scout Car also contains an animation, "tau_levitate," which shows the Tau Cannon detaching from the Scout Car and floating to the front left of it.

The Vortigaunt also features animations prefixed 'TC', which depict the Vortigaunt carrying and firing a weapon, suggesting that at some point, a Vortigaunt was actually supposed to use the Tau Cannon.


  • 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 likely a relic of Half-Life’s early development stages.
  • The tau is the largest 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.
  • The term "Gauss" is derived from the name of the German mathematician Carl Friedrich Gauß who worked partially on magnetism.
  • In the real world, depleted uranium is a name for a mixture of uranium isotopes that contains less than 0.7% of uranium-235 (usually 0.2-0.3%). Furthermore, it is typically very weakly radioactive. Therefore, the name for the ammunition, "depleted uranium-235," and the radiation warnings on the cases do not make any sense in any conventional methods. However, as a hyper-velocity projectile weapon it once was; gauss gun, depleted uranium could have been the projectile launched. In real life Depleted Uranium is also used as a component for armor piercing projectiles ranging in size from pistol or rifle rounds to APFSDS tank shells due to the fact that it is 1.6 times denser than lead.
  • In the multiplayer versions of Half-Life and Half-Life: Opposing Force, if the player charges the Tau Cannon up, the maximum amount of Uranium-235 that can be held is only up to 10, instead of 13 in singleplayer mode, although the charge occurs much faster in multiplayer.
  • Its function and ammo somewhat resembles the Railgun from the Quake series, due to the railgun also firing beams and using depleted Uranium-235 cells.
  • In the Half-Life 2 files, several texture files for HUD weapon icons can be found, most of which still containing the icons for almost all of the cut weapons. The Tau Cannon is among them. It is also present in the font "HalfLife2.ttf," used for all the HUD glyphs.
  • The spinning box of the Tau Cannon is usually spun in a clockwise direction when the player presses and holds the secondary attack of the Tau Cannon. Nevertheless, the player can make it spin counter-clockwise. First, right-click and hold the Tau Cannon until it absorbs the maximum amount of Uranium-235, and then go to the surface of a pool of water nearby, but don't go in the water as it will make the Tau Cannon stop spinning. Look face-down into the pool. If observed carefully enough, the player can see the Tau Cannon is spinning slowly counter-clockwise.
  • Just like the Gluon Gun, despite being a prototype, a second Tau Cannon can be seen in the area previous to the Lambda Teleporter before entering Xen.
  • The cut Half-Life 2 HUD icon is the same HUD icon for the Physics-Gun from the popular game Garry's Mod.





Half-Life 2Edit



List of appearancesEdit


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