Radium Weapons sometimes called Rad Weapons are deadly and highly dangerous weapons that are used exclusively by the forces of the Skitarii Legions of the Adeptus Mechanicus. Think of them as nuclear rifles. Radium Weapons are so volatile that they eventually kill their wielders. Although quite frankly, having a gun that just ends up killing you seems to venture more into Grimderp territory more then anything else. Their baroque beauty belies a singularly vile function; not only to strike, but to render the battlefield as deadly as the rad-wastes of Mars. Each weapon's bullet cylinder is so thoroughly bathed in radium that a volley can cause a localized rad-storm. Those inside such a storm soon find their flesh blackening and sloughing away.
Despite Rad weapons seemingly being Grimdark for the sake of being Grimdark. There is a precedence for these weapons actualizing existing. Energy weapons of all types would throw out huge amounts of Bremsstrahlung radiation each time they are fired. Which is German for "to brake". This mean that even las weaponary, Plasma guns, Melta, Volkites and just every type of energy weapons would be spewing between trace to large amounts. Even high velocity weapons such as Gauss Railguns and certain Autoguns would do this for each bullet,sabot,etc, when fired. As modern firearms have housing to prevent them from overheating. Most weapons by the Great Crusade would have systems in place so they don't cause damage to the user or their armor. Having to clean up even small amounts of radiation would require extensive decontamination of both their gear and the weapon itself. Though the Admech being made up of cyborgs don't care. They can just get themselves a fancy new limb, organ or two. If they survive the battle of course.
On tabletop, generally speaking, Rad Weapons have a low strength of 3 and no AP, but a to-wound of 6 causes two damage. In 30k 6s to wound do two auto-wounds saved separately. A ten man squad without special weapons requires about thirty dice rolls. Normally Ork troops would have that much. While having shootouts with the Tau and can even win. But that's how the Adeptus Mechanicus rolls.
There are three types of Rad Weapons in the Imperium.
A small pistol that uses scaled down radium technology, usually used by Skitarii Vanguard and Ranger Alphas. These nuclear armed hand cannons are unfortunately not as rad (Hah! See what I did there?) as one might think. You're better off ponying up the points for the Phosphor Blast Pistol instead, unless you *have* to have radium. But really you're better off just not taking a pistol on your Alpha.
A rapid firing weapon capable of a punishing rate of fire, it is the main armament of Skitarii Vanguard. If you want to roleplay as Fallout in WH40K, this is the gun for you. Speaking of Fallout, this gun closely resembles the Fallout 4's similarly named Radium Rifle in both design and function. On the table, at 18" range and Assault 3, the Radium Carbine usually means lots of hits at short-mid range.
The basic weapon of the Skitarii Vanguard.
A motherfucking nuclear sniper rifle. The Radium Jezzail is a long sniper-like weapon that is sometimes used by Sydonian Dragoons instead of a Taser Lance. On the table, it's a two shot sniper version as a weapon choice for the Dragoon, in place of its Taser Lance, for free. To Wound of 6's cause an extra mortal.
Basically, if you're going to be taking these on Dragoons you will have to commit to it; as in they're all gonna be sniping.
An Irradiation Projector, also known as a Rad-Cleanser or Irad-Cleanser, are specialized Adeptus Mechanicus weapons. A potent anti-infantry relic of the Dark Age of Technology, the weapon itself is a dish-like projector connected to a bulky generator which unleashes a powerful blast of cross-spectrum radiation.
Victims caught in the blast suffer horrendous deaths as they are boiled alive from within and blasted apart on a cellular level. Though less effective against vehicles or targets with heavy armor, the target will likely die a short time later from late-term effects of acute radiation sickness (compare "Walking-Ghost-phase").
The biggest users are Thallaxes whose enclosed cybernetic harness means that they are less susceptible to radiation poisoning.
Rad Missile Launcher
The big cheese of the Imperium's Rad Weapons.
Rad Missile Launchers were a type of Rad Weapon deployed during the Great Crusade and Horus Heresy. These Terran-derived missiles are a horror of the genetic wars waged during the Age of Strife, using custom loaded-warheads which combine high explosive fragmentation charges lined with radioactive isotopes. So basically they are dirty bombs.
The effect is to create an intensely toxic radiation weapon that inflicts a hideous death on its victim no matter their resilience. Because of its contaminating and hideous nature, it was deployed sparingly, most commonly by Destroyer Space Marines.
Tl;dr, it's a missile launching dirty bomb.
The Tau Empire only utilize one type of rad weapon in which only the Vespids are known to use.
The primary weapon of the giant cockroaches. Neutron Blasters are some type of rad weapon that fires irradiated neutrons. The Neutron Blaster is one of the most bizarre and impractical weapons ever conceived by Games Workshop. This is due to the fact that in order to fire it, you need a constant, ultrasonic tone emitted only by the vibrations of the Vespid wing casings which perfectly modulate the energies contained within the crystals. Whilst some may claim that this allows only the Vespids to utilize these weapons which is a great enemy denial tool. It also presents the problems of the usefulness of this weapon in closed, cramp spaces, or the hard vacuum of outer space which may render the weapon quite useless....whoops....
The Neutron Blaster in a sense, is a hybrid of Vespid and Tau technology, the bulk of a Neutron Blaster is of Tau manufacture as the Vespid have not yet attained the technical facility to fabricate it themselves. Mounted at the barrel of the weapon is a highly energetic and unstable crystal harvested from the lowest levels of the largest stalactite islands of the Vespid homeworld in the deepest reaches of Vespid's cloud seas. At such depths, the atmospheric pressures create all manner of exotic, bizarre and unique crystal formations, and it is only the larger female Vespid, those who form the leader class of the species, who have the constitution to descend to such depths and harvest the purest crystals.
Neutron Blasters are capable of emitting short-ranged but deadly streams of neutron radiation that are able to pass straight through all but the sturdiest of ray-shielded enemy armor to reduce biological matter or delicate machine circuitry to cinders.
For some reason, the Neutron it fires is a sickly green despite the fact that neutrons aren't known to come in green.