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Half-Life Wiki
This subject is from the Black Mesa Incident era.

"Put that down, it's a prototype..."
Black Mesa Scientist[src]

The Tau Cannon, also known as the XVL1456 and/or Gauss Gun,[2] is a powerful particle accelerator with significant stopping power.



The Tau Cannon can be first found in the Half-Life chapter Questionable Ethics, on the second floor of the Advanced Biological Research Lab. If not obtained there, it can be found in the Lambda Reactor Core supply depot at the end of the chapter Lambda Core.

A particle accelerator custom-built by BMRF scientists, it consumes depleted Uranium 235 as ammo and fires devastating beams.

  • The primary fire consumes two ammo per shot and semi-automatically fires singular beams.
  • The secondary fire allows the player to charge the weapon by forcing Uranium into it. When released, it launches a penetrating beam that does significant damage. It also brings forth massive recoil.
    • However, if charged for more than 10 seconds, it explodes, causing 50 damage to the user, though the weapon is left unaffected.

The vehicle-mounted Tau Cannon used during the time of the rebellion against the Combine is a more advanced model in several aspects:

  • It uses an unknown fuel source that can allow it to be used indefinitely.
  • It will not be catastrophically discharged if secondary fire is held for too long.



  • The secondary fire's recoil can be abused to boost yourself in one direction and gain distance or cross large gaps. In multiplayer mode it is also possible to boost yourself upwards.
  • Damage-wise, the primary fire loses out to Gluon Gun since it does 10 damage per ammo spent, while Gluon Gun does 14, so only use it in desperate need.

Half-Life 2

  • This weapon is still powerful, as it can take out combine soldiers or even Xen wildlife in a few hits. Beware that it is still possible to cause friendly fire.
    • However, it is weak against Combine armor since its particles will just bounce off them.

Behind the scenes


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. 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, which will cause a catastrophic discharge if it is held for too long.

Half-Life 2

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 would be able to use the Tau Cannon.


  • The Tau Cannon's nickname in the community ("Gauss Gun") refers to the weapon's codename which is quite literally gauss, which in turn may be a relic from earlier stages of game development.
  • 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, 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 retail, however, it seems to be just supplying the power of the weapon instead and it gains Tau particles from nowhere.
  • 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 as ammunition.
  • 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 rotated 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, hinting that multiple ones were built, or they were placed by the game for balancing reasons.
  • The cut Half-Life 2 HUD icon is the same HUD icon for the Physics-Gun from the popular game Garry's Mod.
  • In Half-Life:Day One in the skill.cfg it has // Tau Cannon sk_plr_tau_narrow1/2/3 "150" and sk_plr_tau_wide1/2/3 "100". This implies as late as when Half-Life:Day one was created they were reworking the tau cannon, what tau narrow and tau wide mean isn't quite known. What's more interesting is that in the final release the game refers to it as gauss gun.





Half-Life 2



List of appearances


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