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. The main drawback is that their are fairly short-ranged, although not to the same extent as Melta weapons.
They are quite literally Martian ray-guns, as only the Tech-Priests 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 grew, 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 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.
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 it's shots caused the enemy to spontaneously combust.
Volkite Carronade
The biggest one of all. 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. 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.