Do LETHAL HITS and DEVASTATING WOUNDS stack in 40k?

Short answer: no, not on the same attack. LETHAL HITS skips the wound roll, and DEVASTATING WOUNDS needs a wound roll to trigger on. Take the auto-wound and you have given up the chance of mortal wounds from that attack.

A weapon can carry both abilities, and across a full volley both will matter. But for any single attack you get one or the other, never both.

Why they cannot both apply

The two abilities sit at different points in the attack sequence, and the first one closes the door on the second.

• LETHAL HITS acts on the hit roll. A critical hit automatically wounds, which means the attack moves straight past the wound roll.
• DEVASTATING WOUNDS acts on the wound roll. A critical wound inflicts mortal wounds instead of a normal wound.

If an attack never makes a wound roll, it can never produce a critical wound. So an attack that auto-wounds through LETHAL HITS is not eligible for DEVASTATING WOUNDS, no matter what else is going on.

Does LETHAL HITS trigger DEVASTATING WOUNDS?

No. This trips people up because both abilities key off a critical roll, so it feels like one should feed the other.

They key off different rolls. LETHAL HITS responds to a critical hit. DEVASTATING WOUNDS responds to a critical wound. An auto-wound handed to you by LETHAL HITS is a successful wound, not a critical one, so there is nothing for DEVASTATING WOUNDS to fire on.

What is the difference between the two?

They solve different problems:

• LETHAL HITS saves you a roll. It converts a critical hit into a wound, so you skip the risk of failing to wound. It does nothing about the target's save.
• DEVASTATING WOUNDS bypasses the save. It converts a critical wound into mortal wounds, which the target does not get to save against.

That difference is what makes the choice interesting. Against a target you struggle to wound, skipping the wound roll is worth a lot. Against a target with a strong save, getting past the save is worth more.

What changed in 11th Edition

In 11th Edition, LETHAL HITS became optional. You can choose to have a critical hit automatically wound, rather than being forced to.

That single word is what creates the decision. Under a rule that forced the auto-wound, a weapon with both abilities would never get to use DEVASTATING WOUNDS off a critical hit. Being able to decline the auto-wound means you can roll to wound instead and keep the mortal wound chance alive.

So which should you take?

It comes down to what you need to roll to wound.

When you wound on 2+ to 5+, rolling is usually the stronger play. You are likely to wound anyway, and rolling keeps a 1-in-6 chance of a critical wound and the mortal wounds that come with it. You risk a small chance of failing to wound in exchange for a real chance at damage that ignores the save.

When you need a 6 to wound, it is a genuine trade rather than an easy call. The auto-wound gives you a guaranteed wound that the target can still save against. Rolling gives you a 1-in-6 chance of a wound, but that wound is a critical one, so it becomes mortal wounds that bypass the save entirely.

Which of those is better depends on the target. Against something with a poor save, the guaranteed wound is worth more. Against a tough save, or a target with a Feel No Pain roll, the mortal wounds can come out ahead even at 1-in-6.

How UnitCrunch handles it

When a weapon has both abilities, UnitCrunch applies a straightforward rule:

• Wounding on 2+ through 5+: it declines the LETHAL HITS auto-wound and rolls to wound normally, keeping the DEVASTATING WOUNDS trigger in play.
• Wounding on 6+: it takes the LETHAL HITS auto-wound, trading the chance of mortal wounds for a guaranteed wound.

A worked example

A weapon with Strength 8 attacking Toughness 4 wounds on 2+. With both LETHAL HITS and DEVASTATING WOUNDS:

• Critical hits (6s to hit) decline the auto-wound and roll to wound normally.
• Most of them wound anyway, since a 2+ succeeds 83% of the time.
• The ones that roll a 6 to wound trigger DEVASTATING WOUNDS for mortal wounds.

The small risk of failing a 2+ wound roll buys a meaningful chance at damage the target cannot save against.

If you want to model the other choice instead, set the weapon up without DEVASTATING WOUNDS, or without LETHAL HITS, and compare the two results side by side.