Volkite Weaponry
Volkite weapons are Martian death-rays.
In Warhammer 40,000.
But before you start crying at the sheer awesomeness of this, take note that they are Horus Heresy-only, so no frying Tyranids with your death rays. Sorry.
Seriously though, Volkite weapons are ancient and advanced weapons based on the principle of imparting so much heat to the enemy that he literally bursts into flames inside his armour. These flaming victims then cause more damage to their own squad, as their run around screaming while emitting the temperature of a rather large and unwieldy furnace. Their main drawbacks are that they are fairly short-ranged (although not to the same extent as melta weapons) and somewhat less effective at piercing armor than bolters.
They are quite literally Martian ray-guns, as only the techpriests of Mars could make them. When the Great Crusade was just starting out, there were enough to equip every single Space Marine Legionnaire with them. Sadly, as the Crusade got on and the Primarchs were found (thus, greatly increasing the numbers of Legionnaires), demand far outstripped supply and Volkite guns were relegated to "special weapon" status. Then, with the Horus Heresy, the defection of large parts of the Mechanicum, and 10,000 years of technological stagnation, the know-how to repair them was lost and Volkite weapons gradually disappeared from the galaxy. But, hell, Games Workshop brought grav-guns back into style with the new Codex: Space Marines. Anything could happen (although that's fairly unlikely, as grav-guns were established in 40k fluff beforehand, whereas Volkite weapons were made up for 30k).
A note on the above, Volkite weapons were excellent assault weaponry, especially when facing massed infantry, and had more potential against light vehicles than a bolter did, but that was pretty much their niche. Bolt weapons, which were much easier to manufacture, having a slightly longer range, and able to use numerous specialty rounds, were a better tactical weapon that could adapt to numerous situations, as well as easier to field in the numbers demanded by Legions. The main drawback in the field was that bolters were most often fitted with standard rounds, while specialty munitions were still rare enough to only be issued to specialist squads alongside their regular bolts.
Named after really old firearms and cannons.
Volkite Weapons
Volkite Serpenta
The pistol of the family, highly effective and powerful sidearm but with obviously low range and rate of fire, and the favoured ranged weapon of a certain Sick Fuck. The Adeptus Mechanicus still have a few of these on hand, but like most of the good stuff, instead of taking it apart and reverse engineering it, they instead keep them locked in stasis vaults and ogle them.
Volkite Charger
The rifle of the family, as stated previous, at the outbreak of the Great Crusade these things were the goddamn standard weapon for the Adeptus Astartes, which I'm sure was a terrifying prospect for the enemies of the Imperium. Unfortunately their laborious construction resulted in them being relegated in favour of the Bolter, which was much easier to manufacture and supply, which was important as the crusade grew from a few thousand marines, a planet, and a moon to many hundred thousands of marines, quadrillions of Imperial Army troops, and an entire galaxy of shit to conquer.
Volkite Caliver
Somewhat comparable to a light squad support weapon, longer and bulkier than the Charger, though still loaded from a box magazine and fairly easy to carry. With a better range and rate of fire than the Charger at a cost of being unable to fire on the move.
Volkite Culverin
The heavy weapon, bulky, powerful, with a massive rate of fire and equally large ammunition requirement. It's like a heavy bolter if it fired even faster and its shots caused the enemy to spontaneously combust. Perfect for that 20 man squad of assault marines.
Volkite Demi Culverin
The bigger brother to the culverin, this is only available to the Solar Auxilla (for now). It boasts one more strength and one more shot than the man portable version and is found on the spiffy Leman Russ Incinerator. Demi culverin. For when you want to say screw that guy and everyone around that guy.
Volkite Chieorovile
Big goddamn Volkite gun mounted onto the Questoris Knight Styrix. With a range equal to the culverin, this baby puts out five S8 AP3 shots which is great at turning entire armies into fire. Who wouldn't want to a gun that kicks out 5 krak missiles that set blobs on fire per turn?
Volkite Carronade
The biggest one of all (maybe; see above). Disgusted by the prospect of the largest weapon in a given type being able to be held by a person, those crazy Martians decided to supersize it and put it on the turret of the Fellblade super-heavy tank. At first glance it's almost comically short-ranged for a super-heavy tank weapon, but when you consider it hits absolutely everything in a straight line (which is as wide as the weapons barrel, which is about the same as the height of a space marine) between its smoking barrel and its max range, turning entire tank columns into molten heaps of slag and reducing infantry formations into screaming, flaming soon-to-be-corpses who proceed to set their own allies alight, it ends up looking pretty good. And unlike with psychic beams, the beam's strength is only reduced by other super-heavies and buildings. That said, it hits YOUR units too, so don't place anyone you don't want to lose in front of it when it fires.