It's like Russian roulette every time you shoot, you survive: you're awesome, you die: well shit happens.
Plasma is the fourth state of matter, beyond gas. It is a sea of ions, or charged particles exposed to so much energy, such as nuclear fusion or lightning, that the bind between electrons and protons become weakened and the electrons freely flow through the plasma as a current just like through a metal. Plasma emits truly heroic amounts of energy, both as heat and light, because the free electrons within the mass are frequently caught in the attractive pull of the nuclei, resulting in circular motion around the nuclei, which decelerates the electron, causing it to emit a photon in order to maintain conservation of energy (Bremsstrahlung emission). This means plasma is incredibly hot and incandescent, though unless it is optically dense (thick enough to reabsorb the vast majority of emitted photons), it cools rapidly. Seeing as it is a mass of charged particles, it is also highly susceptible to influence from electromagnetic fields. Plasma is the most common state of (regular) matter in the universe, this is because intergalactic space consists mostly of low-density plasma, and stars are giant balls of high-density plasma. Nuclear fusion requires hydrogen gas to be heated and charged to the plasma state, where no electrons can interfere with the fusion of "naked" proton nuclei.
Plasma is produced by applying a very strong electromagnetic field to a gas, such as Hydrogen. The plasma is then contained within an electromagnetic field, keeping it out of contact with the weapon's components which it would almost certainly damage. The tokamak, a prototype fusion reactor, exploits this by accelerating plasma in an intense electromagnetic torus until it becomes so pressurized, hot and fast enough that fusion is achieved, although the cost of power outweighed the possible electrical production benefits. Hard sci-fi plasma weapons come in types: "flamethrowers" or cannons that expel stored plasma, Fusion cannons, melta weapons where so much energy is expelled from a weapon that it turns the air outside into plasma, lightning guns such as Tesla coils, with a large pulse of energy sending a huge electrical charge that produces plasma conduits across the air, or Ion Cannon like the ones that GDI uses as its signature superweapon.
In fantasy sci-fi, "plasma" weapons fire insubstantial ammunition that imparts extreme heat on impact, or just as with gauss weapons, plasma guns can be anything that sounds cool.
There is also "blood plasma," which may be of interest to Vampire players, but the idea of a blood-plasma rifle is kinda gross. Pus-rifles? Ew.
 Warhammer 40,000
In the grim, dark, dark, horrible, grim, dark grimdark future, plasma weapons are a type of heavy weapon used by the Imperium, Chaos forces, the Tyranids, and the Tau. Variants are even used by the Dark Eldar and Eldar. They operate by firing a bolt of superheated energy that's hot enough to melt virtually anything in proportion to its size (Pistols and guns can take out heavies while cannons can take out super-heavies and titans), with its primary drawback being the massive amount of heat generated per shot. Imperial and Chaos players like to use plasma weaponry for it's raw, balls-out power and point efficiency.
Plasma technology is relatively rare in the Imperium as it is incredibly hard to produce and maintain properly in large quantities, and is thus scarce in both Imperial and Chaos forces. However, its power means that these forces use it every chance they get. The Tau are so advanced, their basic weapons are plasma-based.
Because of its high firepower, plasma weapons are the go-to choice of most players when dealing with heavily-armored infantry such as Terminators or Tyranid Warriors.
 Imperial/Chaos Plasma technology
Imperial and Chaos plasma weapons are powerful and point-efficient. They are easily capable of taking out light and medium tanks without the need to flank and target rear armour, can go through any infantry armor and are rapid-firing. However, Imperial/Chaos players aren't terribly fond of plasma weapons due to the "Gets Hot" rule. The overheating issue is probably cause by unreliable electromagnetic field generators failing to properly contain the plasma.
According to the fluff, Imperial plasma weapons are extremely rare and most are at least a few centuries old. This is due to the difficulty in mass-producing plasma weapons on any forgeworld other than Ryza, whose hat is that they plasma the shit out of everything, and the difficulty in properly maintaining/using the plasma weapon in light of the massive amounts of heat it gives off.
However, since ALL Space Marine Chapters, many Imperial Guard Regiments, and even underhive gangs seem to have access to them, this admonition comes across a little flat. They might just be extremely expensive and the Adeptus Mechanicus just says that they are "relics" so people will take better care of them. The official reason seems to be they're "rare" in terms of numbers, after a million Plasma guns being made per year would be "small" compare to billions of lasguns made per year. Fact: the Adeptus Mechanicus does have plasma weapon patterns that don't overheat but boggarts them as usual with the good shit. Then again, given how frequent Imperial forces turn traitor from Marine chapters to Imperial regiments, taking whatever post-heresy tech they had with them; the Mechanicus is understandably paranoid about how and what they share tech like this with their allies (or more specifically, how they refuse to share) since their allies are notoriously unreliable and volatile and their main enemy is a malevolent bunch of crazies who're more than happy to reverse-engineer and mass-produce their venerable machines with reckless abandon.
 Plasma Gun
The standard, rifle-sized plasma weapon, capable of a range and rate of fire similar to boltguns and Lasguns, with a Strength value of 7 and an AP of 2.
Stats: S7|AP2|24"|Rapid fire
 Plasma Pistol
The smallest weapon in the family, retaining the incredible power of the Plasma gun, but with a decreased range and rate of fire. As a pistol it also counts as a Close-combat weapon.
 Plasma Blaster
Preheresy relic plasma weapon, that looks like a double barreled plasma gun. In 30K it was used as terminator and dreadnought weapon, but in 40k it only could be found on extremely rare Contemptor pattern dreadnoughts.
Stats: S7|AP2|18"|Assault 2
 Plasma Cannon
The largest man-portable plasma weapon, only used among infantry by Space Marines and Chaos Space Marines, and available as sponson mounts for the Leman Russ Battle Tank. Again, is exactly the same power and armour penetration as the Plasma Gun, AND a lower rate of fire, but rather than firing quick 'pulses' of plasma, fires a charged bolt that, on contact with a solid surface, explodes with a blast of searing deadly plasma.
Stats: S7|AP2|36"|Heavy 1, Blast
 Plasma Destroyer
This weapon is only seen on the 'Executioner' variants of the Leman Russ Battle Tank and Predator Tank, and like the Plasma Cannon, it fires explosive bolts, but in bursts of three at a time thanks to larger capacitors and cooling systems. Unless you go by Dawn of War II: Retribution, then the Executioner fires a lot, very quickly.
Stats: S7|AP2|36"|Heavy 3, Blast
 Plasma Blastgun
The first super-heavy Plasma weapon, found as a Primary weapon on the 'Stormblade' class super-heavy battle tank, and an arm-mounted weapon on the Warhound Scout Titan. Featured as a carapace weapon on Reaver, Warlord and Imperator Battle Titans. An even smaller Omega variant is mounted on the Macharius Heavy Tank for those times when you need to flash-fry entire enemy platoons but can't be bothered to field a Titan. This Plasma weapon features two different firing options: the first is a charged shot with an enormous blast range and apocalyptic power, while the second is a burst of several shots that bathes a slightly smaller, but more flexible area in less powerful (though still devastating) plasma.
Stats: (Charged) S10|AP2|120"|Ordnance 1, 10" Blast
Stats: (Rapid) S8|AP2|96"|Ordnance 2, 7" Blast
 Plasma Destructor
The middle child of the super-heavy Plasma weapons, it ist most commonly found on the Warlord Battle Titan. Like its brothers it has two firing modes, the Rapid mode that lays down several smaller blast templates or the Charged mode that has longer range, deals more damage and uses the larger templates.
Stats (Charged) S10|AP2|96"|Ordnance 2, 10" blast
Stats (Rapid) S8|AP2|72"|Ordnance3, 7" blast
 Plasma Annihilator
The largest land based plasma weapon in existence, only seen on the arm mounts of the terrifying walking cathedrals that are the 'Imperator' Battle Titans. Capable of wiping out entire armies in a single salvo, the Plasma Annihilator has 2 different firing modes, which are essentially the same as the Plasma Blastgun, but at a tripled (!) rate of fire.
Stats (Charged) S10|AP2|120"|Ordnance 3, 10" Blast
Stats (Rapid) S8|AP2|96"|Ordnance 6, 7" Blast
 A Small Notice to all Devou/tg/uardsmen on Plasma Weaponry
Due to a recent modeling discrepancy with man-portable Imperial plasma weaponry (and GW making the errors canon), two things need to be said:
First, the Plasma Pistol, Gun, and Cannon only have one barrel. NO PLASMA COMES OUT OF THE BOTTOM BIT YOU IMBECILES! That is actually the access port to the main accelerator coil of the weapon. The accelerator coil is a ferromagnetic electromagnet, and thus loses magnetic potency as its molecules shift due to the powerful charges and pulses it experiences. This means it wears out with frequency, and needs to be replaced every so often, or at least often enough for the designers to add an access hatch.
Second, handheld plasma weaponry has a very small opening for the plasma to escape from. The large space that everybody seems so fond of drilling out to make an enormous-caliber barrel is in fact a magnetic plate shaped like a shallow parabolic dish.
THE BARREL HOLE IS A TINY OPENING, NOT A GAPING HOLE YOU IDIOTS!!! (You try finding a pin vise that can drill a hole that tiny, genius. Also, soft sci-fi weaponry is SRS BZNESS!) This is because the plasma, when it is being charged, is compressed to what is almost a dense gas in order to increase heat retention. The tiny plasma reactor within the weapon is breached for a quick moment, and the spill is immediately drawn along the length and through a narrow tube in a large, ferrometalic block. The block is supercooled to avoid catastrophic burnout, but it is mainly a focusing and directing mechanism. Through the tiny pinhole runs the plasma charge, but it does not touch the sides due to the block's strong magnetic field. When it reaches the muzzle, it is going so fast and is under so much pressure that the parabolic disk shape is necessary in order to focus the plasma as it nearly explodes out of the barrel. The disk face is also magnetized to avoid touching the plasma. The one exception to this is the latest Pattern of Plasma Cannon, which borrows design elements heavily from the Mars-pattern Plasma Destroyer mounted upon the Leman Russ Executioner. It should be noted that all Mars Pattern vehicle-mounted plasma weapons use the "newer" huge-caliber devices, while Ryza Patterns use the pinhole-barrel devices. This can be seen in the Mars-pattern Plasma Blastguns, and conversely on the Ryza-pattern Plasma Annihilator mounted on the Emperor-class Titan. Perhaps this is part of the reason why Ryza plasma technology does not overheat nearly as easily as Mars-pattern plasma tech?
 Gets Hot
The "Gets Hot" rule, is basically the ONE downside of Plasma weapons, which causes the weapon to overheat to the point where it explodes right on the user's face. If one of your models equipped with a plasma weapon gets a 1 on a d6 on one or more of their attacks during their shooting phase, the model automatically gets a wound that is armour-savable. To multi-wound models, this is a noticeable dent, and to most models, which only have 1 wound, it's potentially deadly. Before 6th Edition, Vehicles were immune to this rule, however, with the advent of Hull Points they now suffer from it, taking 1 Hull point damage unless they save on a 4+.
Still though, a lot of players aren't really concerned with this because of the simple fact that plasma is one of the most easily-accessible anti-armor weapon there is to IG, SM, and CSM players. Besides, it's not like you're rolling a 1 every turn, right? In any event, if your plasma gunner(s) are power armored, you won't care. If they are guardsmen, you have reserves so, again, you won't care if one of them dies.
The Tau make extensive use of Plasma technology. In fact, almost all of their weapons are plasma-based in one form or another. Tau plasma weapons do not suffer from the "Gets Hot" rule as they are less powerful than Imperial models and are somewhat smaller. Note that typical Guardsmen and Space Marine squad typically get no more than 3 special weapons while Tau Fire Warrior teams range from 6-12 men per squad, and pulse rifles have a longer range. So basically, think of a 12-man sniper team that's capable of dropping virtually anything that's not a Terminator in their line of sight.
Most Tau plasma weapons are "pulse weapons," which shoot a particle that super heats the air in its immediate vicinity do to its extreme level of excitement (scientifically excited particle, not derp, so don't comment on that). The Tau also make use of more "conventional" plasma weaponry, primarily mounted on battlesuits and sometimes vehicles. This is closer in function to Imperial models, and consequently has greater stopping power and better ability to cook a target in its armor but shorter effective range. However, the Tau are better at regulating and scaling back the power output of these weapons than Imperial designs, which means that they are not subject to the "Gets Hot" rule but are slightly less powerful as a consequence. Expect any battlesuit-heavy Tau force to be full of these, which is to say, the overwhelming majority of its playerbase.
 Pulse Rifle
The Pulse Rifle is the basic firearm used by Fire Warrior squads, and occasionally Gue'vesa and Kroot who earn the trust of their Tau comrades. In terms of crunch, these things have better range and power than lasguns and even regular bolters. The fluff tends to respect this too, and the first Ravenor novel mentions a Guardsman who (rightly) ditched his own weapon in favor of a scavenged pulse rifle. Whilst they can fire on automatic, they aren't good at it and users are expected to pick and choose their shots rather than fill the air with energy rounds. Unlike what many people think, it does not fire plasma. It shoots individual, energized particles. The same thing as a Disruptor Macrocannon except in fires a single particle instead of a shell of particles. The glow is from the particle hitting the air.
The increased range appears to be due in part to a built-in gyrostabilizer -- that little circle gizmo at the end of the barrel (at least, according to Fantasy Flight Games' Rogue Trader RPG).
 Pulse Carbine
Basically a pulse rifle with shorter barrel, slightly higher full-auto fire range but no single shot long range mode. These usually come equipped with a photon grenade launcher to pin down enemy infantry and markerlights and are given to scouts and bodyguards, or attached to drones. You can choose to give these to your Fire Warriors for free, which is now actually not a bad choice for Devilfish squad.
 Pulse Pistol
Predictably, a pistol-sized pulse weapon. The only time you'll see one of these on the board is if a battlesuit equips an ejection system and the pilot bails out. Hilariously, this thing manages to retain the same shot-for-shot power as a full-sized pulse rifle. In addition the spotter for the sniper drones has one.
 Burst Cannon
Essentially a pulse minigun. These things are on EVERYTHING; stealthsuits, battlesuits, every single Tau vehicle, and if you include Forge World models, aircraft and drones too.
 Plasma Rifle
This is the good stuff. Has exactly the same stats as an Imperial Plasma Gun (minus one Strength point) but doesn't Get Hot. Battlesuits are the only things that can take these.
 Tau Plasma Cannon
Forge World produces a turret for the Hammerhead Gunship that lets it mount a twin-linked Plasma Cannon instead of a railgun or ion cannon. It is basically a quad autocannon with amazing AP. Used for raping flyers, rhinos and marines.
 Ion Cannon
The default non-railgun weapon for Hammerheads, it's basically a long-range Plasma Destroyer with AP3, and the option for a large blast (making it arguably superior to a railgun). It is not specifically called a plasma gun, but seeing as how it shoots a stream of ions at its target, it meets the scientific definition of "plasma." Due to new power source (and new codex) it could be overcharged for single S8 AP3 pie plate with gets hot, to rape some marines even more. In fact, considering that most, if not all Imperial plasma technology uses "ionized plasma", there should be no difference between an Imperial plasma weapon and a Tau ion weapon. Given that Imperial tech is specifically stated to be more powerful than other factions' equivalents, though usually less refined and less sophisticated, an Imperial plasma cannon should be significantly more powerful than a Tau ion cannon. Speaking fluff, by the way, not crunch. Oh well, sci-fi writers and science...
 Ion Rifle
Aforementioned new power source allowed Earth caste engineers to create tau-portable ion weapon. Resulting weapon combine the size of pulse rifle and firepower of imperial autocannon in one deadly package. Like it's big brother, ion rifle could be overcharged for S8 AР4 small blast, but in order to overcharge his rifle soldier must expose rifle's power source to air, which is not a good ideam considering said power source is highly radioactive. As result, pathfinders, equipped with this weapon usually dies from cancer, unless they got killed in action before (and considering high risk of their job, there is a good chance they would).
 Ion Accelerator
This unholy piece of cheese is one of the primary weapons available to
RipRapetide battlesuit, and is designed to rape TEQs, especially multi-wound ones (i.e. Paladins and Meganobs). Draigowing, Deathwing and Loganwing armies shit themselves and run away crying like a little babies when they see a pair of giant weaboo robots with these guns.
While limited as the Eldar prefer the more stable and accurate laser-based weaponry, they still makes use of a few plasma-based weapons.
The Eldar Starcannon fires plasma bursts, and does so by firing many smaller pulses as opposed to single stronger ones. Starcannons are more accurate and infinitely more stable than their Imperial counterparts, and are typically seen mounted on Falcon and Wave Serpent Grav Tanks, and weapon platforms used by Guardian Squads.
 Plasma Grenade
Plasma grenades are basically plasma in grenade form. They're a small but deadly time-delayed explosive device that releases a plasma-based explosion after a short amount of time, much like the Imperium's frag grenades. Despite their description and obvious anti-everything application, TT-wise they can allow their assault forces to ignore the attack initiative their target get when their opponents are in cover and be thrown/attacked with in combat like other grenades. Since units with these are generally armed with guns with ridiculously short range anyway, could this be called synergy? So yeah, it's basically
an overpriced flash grenade in the 40K universe AP4 and one of the two positive effects on the current Eldar in 6th edition.
 Disintegrator Cannon
Plasma weapons are often described as striking with the force of a sun. Because of the pathological need of the Dark Eldar to make everything dark and edgy, their Disintegrator Cannons are powered by a particle of matter of a stolen sun. So yes, not only can they steal entire stars and proceed to weaponise them, they do so at the cost of the sheer power that the Imperial designed weapons have. The plus side is that these weapons do not overheat, fire faster than any comparable design and still strike with enough force to vaporise a Space Marine.
The Orks have only one plasma weapon, but that's all they need. It comes in many different shapes and sizes.
 Kustom Mega-Blasta
A while back, some random Mek took a plasma gun from some dead Imperial and thought to himself "We needs us sum ov dese." Today when they need something to kill all those pesky MEQs and TEQs, Mekboys can carry a S8 AP2 Assault 1 plasma slinger. It can also be mounted on Grot vehicles. Unfortunately, due to most Orks' shitty armor saves combined with Gets Hot, the second option is more common.
Kustom Mega-Blastas do not always have to fire plasma, although they are used in a tactically identical manner. Kustom Mega-Blastas could be Microwave lasers, lightning guns, linear accelerators, or even micro-wormhole generators! Modeling on a GW-produced Big Mek with Kustom Mega-Blasta (that sold like spoiled cheese) showed the Blasta in question to be a bulky pistol with an ammo feed, so a fast-firing pulse-weapon is not out of the question either. Either way, plasma is a common variant, and Orks just make and do whatever they want.
The Tyranids makes use of "Bio-Plasma", which they vomit out through their mouths. To do so, a Tyranid with this ability regurgitates an unstable series of chemicals from its innards, which it then spews from its mouth, ignited using electrical impulses along the Tyranid's throat - much in the same way that a Venom Cannon works. Once launched, the glob of biochemicals go off in an incandescent explosion of plasma. Because it does not burn while inside the creature it will not overheat inside its user, negating any heat related issues. It's not terribly popular as far as upgrades go since players in general prefer Venom Cannons, which are both safer and longer-ranged for their anti-armor purposes, though Bio-Plasma can prove useful against heavily-armored infantry.
The Exocrine is a Tyranid creature that comes equipped with a huge Bio-Plasma Cannon which can fire either a stream of the stuff to hit several targets or a great ball of it to cover a small area with Marine-melting goop.
 A Weapon that Beats Spess Mehreens? NEVAR!!
|This article or section involves Matt Ward, your Spiritual Liege, who is universally-reviled on /tg/. Because this article or section covers Ward's copious amounts of derp and rage, fans of the 40K series are advised that if you proceed onward, you will see fluff and crunch violation of a level rarely seen.|
When you have a weapon that's good at killing Marines, it's only a matter of time before you draw the ire of the spiritual liege. Infuriated that there were units in the 41st Millennium which actually had the firepower and weaponry to pop asshole units like Terminators, Matt decided it would be a lovely idea to give the Grey Knights a piece of Wargear for the Ordos Xenos inquisitor that functionally renders any unit using a Plasma weapon within 12" of that unit completely worthless, as its plasma weaponry is treated as BS1. The Ulumeathi Plasma Syphon is supposedly hindered by its short range, but in practice, /tg/ has found it quite easy to get the Inquisitor carrying it into the enemy's midst (often with allied units protecting it) in order to keep high-threat units like nests of Fire Warriors or a Leman Russ Executioner completely locked down. It's particularly nasty against Chaos (which as a rule lacks high-AP weapons except for the Vindicator's Demolisher Cannon and Plasma Weaponry) and Tau, who rely extensively on plasma weapons as described above. Apparently Ward forgot that there is a difference between fluff making Marines unstoppable juggernauts and actually making them that way on the tabletop. 6th Edition ameliorated this somewhat with the advent of the Inquisition Codex giving more Imperial Armies (and a handful of xenos armies as well) access to this broken tool and making the thing only affect Plasma weapons as described in the BRB, thus giving the Tau a fighting chance. Everyone else (Chaos as a whole, basically), though, is still right fucked.
Fuck you, Matt Ward.
But still grav-guns.