The way GURPS stats HEAT rounds and other shaped charge weaheads is among the most realistic I've ever seen in a table top RPG.
To bad no one seems to know how they work...
Well, it's time for me to bust out the rules and explain how they do once and for all.
Over on his blog Chain Link and Concrete Michael Eversberg (the second!) did an excellent write up of an Iranian copy of a Russian missile, the HY-2 Seersucker. And yes, I do find it awesome that he made the distinction between it and Russian version (which is often incorrectly called Silkworm, that's the NATO reporting name for the Chinese version dang it!).
While talking with him in the towel section of post he told me he wasn't 100% clear on just how HEAT worked and this seems to the a common occurrence among players.
And to be fair, I don't blame them. The rules for them have changed three times!
In the Basic Set, thing were simple enough. As it is explained on note 7 on pg 281 and in more detail on pages 414, HEAT rounds had an 10 armor divisor and did crushing explosive damage. Whatever was directly hit took the damage and had their DR divided by 10 and everything else took damage as if hit by a normal explosion. If an attack happened passed its 1/2D range, the 10AD was lost.
This was simple but it was the wrong way to model how shaped charge weapons worked. This model greatly inflated their blast effect and made it hard to model things like HEPD rounds which are HEAT rounds that trade off a small bit of penetration power for a larger anti-personal blast effect. Also there is no reason that modern HEAT rounds would lose there AD (however earlier HEAT did have a tendency to hit off angle at long range but that wasn't an automatic thing).
This changed in Ultra-Tech. As show on Pg 154 and pg. 155 (as High Explosive Multi-Purpose), now HEAT rounds had two parts, an crushing incendiary attack that had an 10 armor divisor which was then linked to a crushing explosive blast. This was a more complicated way of handling it but closer to how they really worked. However, no in-depth explanation is given in the book its self.
Now this version makes a good deal sense, at lest in the context of how GURPS rules works, and even thought the book didn't give an in-depth explanation it was easy to figure out based on the stats. The crushing incendiary attack modeled the penetrating jet of a HEAT round and the linked blast modeled the actual explosion of the HEAT round. The target took the crushing incendiary attack and any DR the target had was divided by 10 while anything within 2×dice of damage yards of the target would take damage from the linked blast. If you were handling what happened to people and items inside of the struck target then the Occupants and Vehicle Damage rules found on pg. 555 of the Basic Set apply.
This was again changed slightly in High-Tech and are covered on pg. 170 (which covers HEAT, HEDP and MS-Heat) and shaped charges as demolitions tools are covered on pg. 182-183. The crushing incendiary attack was changed to a crushing explosive attack for some reason and was still given a linked explosive blast. Once again, no in-depth explanation is given in the book on how you rolled the damage.
So yeah, how do you treat that primary attack now? It's now treated as an crushing explosive so it should act like any other explosive with an armor divisor right?They somehow now did two explosions right?
Even though it's called a crushing explosive attack you do not treat it as such (at lest not to things around the target)!
So yeah, pretty confusing.
So how do you handle a HEAT rounds damage then? If the books don't full explain how you rolls the damage and every one seems to change the rules, what are you do?
You call a doctor!
In this post on the GURPS forum, the indispensable DR. Kromm explains how they're bloody supposed to work! And in this post he further explains that primary crushing explosive damage of a HEAT round only affects inside of the target and therefor replaces the Occupants and Vehicle Damage rules!
After you give those post a read, come back and let me walk you through this step by step.
For our example let's use a 40mm HEAT rounds that does 5d×4(10) cr ex with a linked 4d cr ex blast.
Step 1) The main damage. Roll the 5d×4 and divide any DR the target has by 10. Apply the Penetrating Damage done to the target as crushing for figuring out the amount of Injury the target receives. DO NOT roll damage for other targets with in its blast radius of 2×dice of damage or 40 yards! For all intents and purposes ignore the explosive modifier on the crushing damage for targets outside of the target. Equipment and personal inside of the target however would would take whatever damage made it through the targets DR as an explosive attack. Convert the penetrating damage back to dice and anyone with 2×dice of damage of the point of impact would be subject to attack (though to speed up play, unless it's PC in danger I would just apply whatever damage got through to each target and skip the whole convert back to dice step) this super secedes the the Occupants and Vehicle Damage rules.
Example: I roll 5d×4 and get 64. The target has a DR of 550 which is reduce to 55 by the attacks AD10, the means 64-55= 9 points of penetrating crushing explosive damage which gives gives the target 9 points of injury. There is no further effect on targets outside of the target. ignore the normal the Occupants and Vehicle Damage rules and instead anyone within the target however would then be vulnerable to get hit by the remaining 9 points of damage, which can be treated as an attack doing 2d+2 to anything with in 4 yards of the point of impact.
Step 2) The linked damage. This represents the HEAT rounds actual explosion. Everything within 2×dice of damage (or 8 yards) that somehow can't dive behind cover or out the range is hit for 4d worth of explosive crushing damage including the main target. Roll for separately for each target and for every eligible target outside of the targets hex, divide the damage by 3×distance from the targets hex in yards.
Example: With in 8 yards of the targets is the target its self and a solider standing 3 yards away. I roll the linked damages 4d against the target and get a 9, the blast harmlessly bounces off its DR550. The solider rolls to try and duck and dive to negate the effects but fails and I roll 4d and get a 18 which is then divide 3×3 yards or 9 reducing it to 2 points of damage. The solider has a combat vest on which gives him DR3 vs crushing damage but since this is large-area injury his DR is treated as the average between his most and lest protected area expose to the attach which is 0 he only gets DR3/2 which is effectively DR 2, his vest just barley saves him from injury!
So to recap, treat the main attack as just crushing damage against the target that is hit and ignore the explosive modifier. Treat it only as an explosive attack for things inside of the target. For the linked blast, this is handled in every way like a normal explosion.
Still confusing? Yeah. Do I think it makes sense how this was handled? Technically yes. Is it frustrating that it takes posts on the forum that not everyone who plays is either going to know about or have access to, to explain how this works? Yes.
This is one of the few times I preferred how Ultra-Tech handled things. By making the primary attacks damage crushing incendiary it helped prevent a lot of this confusion from happening. The internal effects can be handled by the normal the Occupants and Vehicle Damage rules. But regardless of how I feel, this is the official ruling.
Alright guys, hope this clears things up for ya. Till the next one.