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The Tau Cannon, also known as the Gauss Gun, is an experimental energy weapon introduced in the Half-Life chapter Questionable Ethics, on the second floor of the Alien Research Labs. 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.
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 by 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. 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 you backwards high into the air, propelling you 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. The prototype and its materials somehow survived the facility’s destruction and years later the Tau Cannon was utilized by the Resistance, including a version that was mounted on the Scout Car that was given to Gordon Freeman.
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 in Half-Life 2 is a much more advanced model. It uses an alternative fuel source that gives it seemingly unlimited ammunition (Perhaps it gets its power from the vehicle's engines or the air), and the capacitors can hold a full charge indefinitely, so the player doesn't have to worry about catastrophically discharging the weapon. The weapon is devastating against Combine soldiers and alien life-forms, but useless against Combine APCs as their advanced armor is designed to reflect energy weapons.
 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, but did not feel there was any need to change names of the game’s data files to reflect this. 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 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.
Four unused closed captions for the Vortigaunt removing the Tau Cannon from the Scout Car can be found in the Half-Life 2 closecaption_english.txt file.
The Vortigaunt model has unused Tau Cannon animations labeled TC. This means that Vortigaunts were originally to use the Tau Cannon.
The Scout Car also has an unused animation depicting the Tau Cannon hovering off of it's mount into the hands of someone standing alongside the car, predictably the aforementioned Vortigaunt.
- 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.
- 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.
- 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 "gauss ammo icon" placeholder glyph can be also found in the font, similar to the Stun Baton's icon.
- Animations exist for Gordon manually spinning the capacitor, which trigger when the weapon is idle.
- XVL1456 is written on its side in the High Definition version of the viewmodel.
 Half-Life 2
 List of appearances
- Half-Life (First appearance)
- Half-Life: Opposing Force (Multiplayer only)
- Half-Life 2 (As mounted on the Scout Car only)