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Are EMPs a weapon used in the 40K Universe? If so, how common is it? Will it work against mostly anything? I can see detrimental damage to army factions like the Iron Hands, Adeptus Mechanicus, Necrons, etc. Thoughts?

Is that what Haywire weapons essentially are?

Haywire weapons are basically EMPs. You see them used by a few aeldari specialized units and as less specialized forms of aeldari grenades. The mechanicus also has weapons that had the haywire USR when it was still a thing including the arc rifles on their basic troops. Tau have at least one example of an EMP weapon on a special character, and I think they had EMP grenades for a while.

They probably *should* be very effective against Iron hands, necrons, and anyone in power armor really (there's a short story set in the Horus Heresy book Primarchs in which we see a sort of mass EMP being used against Iron Hands), but that's hard to balance and was previously difficult to put into game turns.

Like, haywire blasters for drukhari (found on scourges) are meant to be the dedicated anti-tank gun. They're less of an all-rounder than things like blasters and heat lances, but they've always had special rules that made them very reliable against vehicles (like hurting vehicles on a 2+ last edition, or doing mortal wounds against vehicles on a 4+ this edition).

But last edition, when keywords weren't a thing, it's tricky to explain exactly what units are and are not effected by those special rules. Saying, "vehicles" is easy. "Vehicles, bikes, necrons, iron hands, and maybe units with a variation on this type of armor," less so. Plus, if that "specialized anti-vehicle" haywire blaster happens to wound your opponent's entire army on a 2+ in the right matchups, how do you set its points cost?

In the dark heresy RPG it was pretty common to get limbs blown off and get cybernetic prosthetics. Lets just say that one did NOT want to be in a haywire field (read EMP) with cybernetics. IIRC there was a youtube campaign where the guys replacement lungs just stopped working for a bit.

Honestly, i'd expect necron tech to be so outside anything imaginable an EMP wouldn't be worth a fart against it. They might not even feel it.

"Cryptek, that's the fifth time these primitives have detonated one of those strange devices? Have you any idea what they are?"

"They appear to be designed to produce a brief spike in electromagnetic radiation, Warlord."

"Electromagnetic radiation? Similar to their crude communication devices? Are they trying to communicate with us, or summon more of their kind?"

"I..do not believe so, Warlord. The employment of their devices would limit their communication range due to line of sight issues, and they seem to mount attacks against us with renewed vigor immediately after each detonation."

"Then do you have any notion what these seemingly pointless devices are? More of their inane religious practices?"

"Warlord, I have noticed that in some cases human war machines, if such they deserve to be called, have been near the detonations of these devices, possibly due to human inaccuracy and imprecision. In such cases their pathetic technology seems to falter, or even fail. It is my considered opinion, warlord, that the EM pulses these otherwise useless devices produce may disrupt the operation of their incredibly base technology, and they possibly believe they could affect our advanced war machines, or possibly ourselves, in a similar way. That is my best theory at this time."

"These humans, they are little more than cave dwellers! I would be insulted by their belief that these laughable devices could harm us, were I not more amused at their unfathomable ignorance!"

Vilehydra wrote:
In the dark heresy RPG it was pretty common to get limbs blown off and get cybernetic prosthetics. Lets just say that one did NOT want to be in a haywire field (read EMP) with cybernetics. IIRC there was a youtube campaign where the guys replacement lungs just stopped working for a bit.

Played dark heresy a lot back in the day. As I recall the most effective way to deal with machines was actually psychic powers. You could mess with machines in a variety of ways. Like jamming weapons in a radius, make all machines in an area stop working and so on. I think there’s rituals in black crusade that let you mess with machines as well. Wasn’t there a “curse of the machine spirit” psychic power in early 40k editions, like 3rd/4th? Since then 40k has moved away from psychic powers messing with machines but it was very much a thing back then.

Seeing as even today it's pretty common to use Faraday cages (even a microwave is one), I think it would pretty useless against marines, I would say necrons too. I think orks would be pretty vulnerable to it though

 Matt Swain wrote:
Honestly, i'd expect necron tech to be so outside anything imaginable an EMP wouldn't be worth a fart against it. They might not even feel it.

"Cryptek, that's the fifth time these primitives have detonated one of those strange devices? Have you any idea what they are?"

"They appear to be designed to produce a brief spike in electromagnetic radiation, Warlord."

"Electromagnetic radiation? Similar to their crude communication devices? Are they trying to communicate with us, or summon more of their kind?"

"I..do not believe so, Warlord. The employment of their devices would limit their communication range due to line of sight issues, and they seem to mount attacks against us with renewed vigor immediately after each detonation."

"Then do you have any notion what these seemingly pointless devices are? More of their inane religious practices?"

"Warlord, I have noticed that in some cases human war machines, if such they deserve to be called, have been near the detonations of these devices, possibly due to human inaccuracy and imprecision. In such cases their pathetic technology seems to falter, or even fail. It is my considered opinion, warlord, that the EM pulses these otherwise useless devices produce may disrupt the operation of their incredibly base technology, and they possibly believe they could affect our advanced war machines, or possibly ourselves, in a similar way. That is my best theory at this time."

"These humans, they are little more than cave dwellers! I would be insulted by their belief that these laughable devices could harm us, were I not more amused at their unfathomable ignorance!"

Exalted for accuracy.

The Tau Icefire Warhead stategem in Apocalypse Reloaded inflicted a pinning test on Necrons in addition to the standard effect.

Necron Warriors get hit by a haywire grenade in The Vorago Fastness and are outwardly unaffected, although their weapons are disabled for a time.

Way back in 2nd edition, a Haywire Grenade would be Strength 3 and bypass armor saves against Terminators (back when it was 3+ on 2d6). Against vehicles it was an auto d3 on the damage table, and if I recall correctly, it also destroyed any targeters caught in the area of effect. That might not seem too bad except it was a one use wargear item taking up a limited wargear slot.

 NinthMusketeer wrote:

 Matt Swain wrote:
Honestly, i'd expect necron tech to be so outside anything imaginable an EMP wouldn't be worth a fart against it. They might not even feel it.

"Cryptek, that's the fifth time these primitives have detonated one of those strange devices? Have you any idea what they are?"

"They appear to be designed to produce a brief spike in electromagnetic radiation, Warlord."

"Electromagnetic radiation? Similar to their crude communication devices? Are they trying to communicate with us, or summon more of their kind?"

"I..do not believe so, Warlord. The employment of their devices would limit their communication range due to line of sight issues, and they seem to mount attacks against us with renewed vigor immediately after each detonation."

"Then do you have any notion what these seemingly pointless devices are? More of their inane religious practices?"

"Warlord, I have noticed that in some cases human war machines, if such they deserve to be called, have been near the detonations of these devices, possibly due to human inaccuracy and imprecision. In such cases their pathetic technology seems to falter, or even fail. It is my considered opinion, warlord, that the EM pulses these otherwise useless devices produce may disrupt the operation of their incredibly base technology, and they possibly believe they could affect our advanced war machines, or possibly ourselves, in a similar way. That is my best theory at this time."

"These humans, they are little more than cave dwellers! I would be insulted by their belief that these laughable devices could harm us, were I not more amused at their unfathomable ignorance!"

Exalted for accuracy.

I got one hell of a laugh ou of this, it really made me feel better than it should have, which is just a testament to how down I am now. Thanks for this.

Despite supposedly having protection from it, Tau used EMP to disanle power armour at thr end of one of their novels.

It was a Phil Kelly novel, so obviouslu theres going to be a lot of Tau spank in there