I’ve always had the impression from the books (well Frank’s, as I haven’t read that many of the newer books) that any lasgun / shield interaction will result in an atomic reaction.
I was a bit surprised when p182 in the core book says (under fortifications) that house shields are powerful enough to be impervious, even to lasguns.
Is this correct? Do you not get an atomic reaction if you point a lasgun and shoot a house shield?
The Lasgun/Shield interaction is a bit of a mess when you try to actually implement it.
Per the book, yes. Lasgun/Shield interactions cause a sub-atomic fusion explosion at one or both of the end points.
Modiphius reiterates the Lasgun/Shield interaction on pg 23 (Everyone is killed by the Imperium if they try it). I assume the additional comment on the House Shields is to stop players using a remote lasgun fired at a fixed location to immediately wipe out another House.
It is a fudge to make things work in game when you have players.
Dune: Awakening plans to get around it by having Lasguns have a safety function that automatically cuts out when it is pointed at a shield.
I’ve not found it saying that under fortifications, can you quote the passage?
Generally the lasgun won’t penetrate the shield. But it won’t need to as there will be a cascading nuclear explosion from the point they hit. 
P182, first column.
“Fortifications are stationary defenses, such as walls and obstacles, that prevent an enemy from moving into or through an area, or which provide cover and protection for defending troops (or both). The simplest kinds of fortifications are earthworks which can be hastily constructed by troops in the field, while the most potent are House shields that can protect a palace or castle from attack. Such shields cannot be pierced by slow-moving objects—a castle can have life support systems within to sustain the inhabitants while the shield is up, so the shield doesn’t need to let air pass through— and are powerful enough that they’re impervious even to lasguns. Obviously, the more powerful the fortification, the more costly it is to construct or obtain, and fortifications cannot be moved.”
Emphasis mine.
Ah, that second reference was edited out in the updated version of the rulebook so consider that a typo.
A shield is strong enough to withstand a blast as powerful as a lasgun, but an actual lasgun has a different effect.
I’ve created a table to be rolled on if lasgun meets shield, to make it even less of a choice to players. Depending on distance there’s a chance that the target will blow up, both target and attacker will blow up, just the attacker will blow up, and the size of the explosion is random too. 
That’s how I always thought it worked in the books anyway.
As for the point that nobody does it because the Imperium will invoke the Great Convention, unite and wipe out anyone who tries that trick, it seems to me that would only happen if the Imperium knew about it in the first place. Which naturally creates the incentive to utterly wipe out an enemy to the last man, woman and child so that there won’t be anyone to tell what happened.
Yup, absolutely.
But you can amend it for your game. Apparently the Dune Rising MMO has been allowed to have people firing lasguns wherever they like. Be interesting to see where that goes.
I always wonder how a 0.1 solaris laser pointer will interact with a very expensive personal shield.
And never forget that glowglobes and personal suspensor use Holtzman effect if you need a complication.
Indeed, in the dune books, we talk about an atomic explosion when a lasgun hits a shield. On the other hand, during the books, particularly in those of Brian Herbert, Frank’s son, the soldiers often have lasguns and use them often. On the other hand, at no time do we explain how they know that their target is not wearing a shield and this situation almost never happens.
There are several logical problems with applying the Holtzman effect and lasguns too categorically.
Firstly, a head of state (or house) would never allow his soldiers or officers the possibility of triggering an atomic explosion, even if only by accident since the political consequences of such an explosion are the annihilation of the House. During battles, soldiers are confronted with various situations, including fear, injuries, trauma. Sometimes, they are injured, sometimes, we give them pain medication which can affect their judgment, sometimes, we give them drugs to counter fatigue or to put them in a state of violence, etc. Not to mention that some soldiers are sadists or extremists and if they have just lost loved ones, they may want revenge more than following the rules of their house.
In such a circumstance, we would never distribute lasguns as any soldier’s primary weapon. These weapons would be managed like atomic weapons, with very strict activation procedures and very precise and binding rules of engagement.
Second, the problem with the Holtzman effect is the creation of a cheap form of nuclear explosion that allows everyone to create a lot of destruction for inexpensive material. The big houses have family atomics, but the poor houses just have to shoot the shields with their laser to do the same.
Third, it is enough to send a suicidal man shot on his own shield to accuse the other house of having used atomic weapons and let the Landsraad make it banned.
So, fourthly, the risks listed here would probably push the Imperium not to condemn the Holtzman effect, but rather the use of lasguns because they are too risky.
To solve these problems, I modified things a little. The Holtzman effect as known was with the first models a long time ago. Now, with the new shield and lasgun models, there is a risk, but it is now minimal but still real. A large atomic explosion could be possible between a shield and a laasgun if the shooter is aware of the intensity, the frequencies of the shield, but for that, he must have spies who tell him about the current calibration of the shield. Such knowledge can increase the risk of an explosion, without guaranteeing it.
Without this knowledge, the chances of an explosion are possible, but would mainly fall on the shooter and not the target. The greatest risk is not an atomic explosion, but damage to the weapon or its explosion of the weapon which will injure the carrier.
My goal is to remove the risk for the defendant to put the danger on the shooter. The goal is that the probability of a risk to the shooter is sufficient to be dissuasive, but not enough to be able to derive a tactical advantage and to be able to use the Holtzman effect as a high-quality atomic bomb.
The idea of putting a pre-laser which analyzes the presence of a shield and which prevents a lasgun from firing a laser without cel is excellent. This optin can be removed, but this puts the shooter in danger and not the target. THE greatest risk is a local explosion or destruction of the lasgun.
You can had that small weapons and shields create mini explosion that destroy the shooters. Only large shields hits by by very large warship lasgun produce atomic explosion. And those explosion occure on the shooter side.
Big lasguns are powerfull enough to destroy personal shield, but at the risk of damaging the big lasguns wich cost lot of money. Small lasgun shooting large shield increase the chance of the explision of the lasguns.
Like i said, use the chance of explosion as a reason to avoid having character shooting at shield with lasgun. But put those chance low enough that trying to create a delibarate explosion are not good enough to be use as a strategy. But if you realy need a holtzman effect explosion in your game, explain how the lasgun has been perfectly calibrated with the intensity and frequence of the shields.
Go for simplicity. A complication of lasgun vs shield is weapon Destructions or character wounded, maybe anyone in the zone if you want to make a point to the player and if they eant to abuse the effect, nothing happen. Just made your player know that whatever they want to achieve with lasgun vs shield is not worth it.
I’ve looked in Dune and there are three instances where they talk about the Lasgun/Shield interaction.
First at the dinner party where Jessica thinks about how the Harkonnens have been smuggling Lasers onto Arrakis which she believed meant the Harkonnens were not bothered by the Shield/Lasgun interaction.
She then continues by considering that Paul will be sleeping far below ground level as a precaution.
The next incident is where Duncan plants a shield for the attacking Saudakar who are using Lasguns with apparent no qualms. Despite Duncan having his personal shield and checking to see if Paul needs one as well.
Jessica initially mistakes the explosion for the House Atomics. She seems shocked, but Paul immediately deduces Duncan used a Shield to trigger it, so clearly it is not an unthinkable tactic.
The final occurrence is right after Paul uses the family atomics to open the shield wall (Presumably a more contrable explosion than more shield/lasgun interactions). The Fremen engineers are using Lasbeams to cut through the rubble and Paul warns them of the danger of active shields. What that tells us is that Lasers are used in industrial applications as well as military ones. Which suggests that they are fairly common, if not as long ranged as the weaponised version.
Overall it is a tangle that is hard to resolve if we keep it as it is.
Personally I like Aralkar’s suggestion of shift the emphasis to the shooter, from the defender.
I’d be happy to have bigger booms, like seen with Duncan’s boobytrap, but it makes it harder for players to abuse as easily.
A threat expanse can switch the side that will explode…
When my players make dubious plans, many of them including explosions, and ask “Is the plan safe ? Can it explode?” I usually say "It looks safe; unless I decide to spend threat. The fear of threat is more useful that the spending of threat…