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Friday, August 22, 2014

Critical Hit Effects in GURPS

I've altered the effects of critical hits in GURPS to remove what I perceive to be the unrealistic aspects. I've tried to give results that can be explained as physically happening in the game world.

I think I need to begin by looking at what critical hits are and why we have them in the first place. As I understand them, their purpose is to give us a mechanical way of representing outcomes that are otherwise below the game mechanics' level of resolution.

Critical hits have two effects:
1) They remove the defender's chance to defend against the attack.
2) The attacker makes a roll on the critical hit table and gets a supposedly physically possible effect with a low rate of occurrence.

In regards to #1: I don't see how the critical hit rates would differ based on skill. I'm not familiar with earlier editions of the game, but I imagine this was acting as a pseudo-deceptive attack before that rule existed. That skill is tied in with #2 seems extremely odd to me, though. I can't justify highly skilled characters experiencing those essentially random and uncontrollable outcomes more often. It seems strange that a critical hit, something external to the defender, removes the defense roll entirely. It's difficult to explain how a robot with reflexes a thousand times faster than a human's can fail to defend against one attack in eleven from a human swordsman, regardless of the swordsman's skill.

Now my major concern: The critical hit table in GURPS gives results that don't make any physical sense. What does damage in GURPS represent? Kinetic energy. It's what determines whether an arrow pierces a bronze cuirass and it's what determines whether a bullet pierces a tank's glacis. What, then, do the double and triple damage results on the critical hit table represent? Clearly they are an increase in kinetic energy. I find this incompatible with reality. Ten damage is 625J, nearing the upper limit of human muscles. Thirty damage is 5625J, which is well beyond anything the human body is capable of. A bow or gun, or a human's sword arm, don't function this way. I can't conceive of this result ever happening in an unpredictable manner. If my gun occasionally shoots for triple damage, then clearly I should be able to alter its mechanism so it always shoots for that amount of damage. If you look at Olympic athletes and the results they get at events, their muscle output is consistent, with variations of one or two percent. If human muscles, guns, lasers, and all other weapons outputted five or ten times their normal wattage as GURPS claims they do, this would show up in measurements of athletes and firearms.

The ability to ignore some amount of DR also makes little sense. It can't be explained as a flaw in the armor in anything other than a narrativistic sense; what if my armor really has no flaws? Does the critical hit travel backwards through time and cause the smith to work weak spots into the armor? Weak spots that can't be detected by even the most advanced instruments (for the DR of homogenous TL12 robots and vehicles are also halved).

Why would a TL11 war robot drop whatever it's holding because it was struck by a man with a sword? Why would this continue to happen after that model's next firmware update?

I don't believe these results are intended to explain the physical goings-on of the game world itself. The results they provide can't be explained in terms of the physics of the in-game universe. When using these rules, I am unable to achieve the desired sense of verisimilitude.

As such, I offer these alternatives:

The rules:

Critical hits no longer remove the defender's chance to defend. I am considering three options here. Option #1: no defense penalty, defender defends as normal; Option #2: defender defends at -4; Option #3: defender defends at half of the attacker's margin of success on his attack roll.

I think I prefer #2 here for ease of use, but I could argue for either of the other two.

And here is my new critical hit table:

3 You may change the hit location of your attack to any other of your choice.
4 Maximum Damage.
5 Your opponent can't retreat from this attack.
6 Shock penalty from attack lasts an additional 1d seconds.
7 Attack is at an additional -2 to defend against.
8 You get +3 on defenses against the attacked target until your next turn.
9, 10, 11 No additional effect.
12 Same as 8
13, 14 Same as 7
15 Same as 6
16 Same as 5
17 Same as 4
18 Same as 3


  1. I always interpreted double and triple damage as a perfect combination of the strike hitting the opponent at the worst possible moment - when the defender's momentum is coming forward into the strike, the hit location is turned to its most vulnerable point, etc. - that everything that needs to happen for the maximum possible effect happens. It's a rare roll anyway, and it helps explain those unlikely but possible effects of strikes that are well outside of the expected range (aka, normal damage range).

    Half DR, same thing - finds a weak point, hits the armor as its layers are shifting apart, hits those spots armor doesn't cover well (I can't think of a heavy real-world armor that universally armors everything the same way), hits a "shot trap," hits a manufacturing flaw (uniformly perfectly formed surfaces are very rare), etc.
    In the past, for certain situations, I've re-rolled this because it makes no sense (say, vs. a DR 10 creature made entirely of DR 10 material)

    5 and 7 don't make sense by the rules as written - critical hits already allow no defenses, so those situations can't come up.

    And what's the rationale for 9? You hit so well you are in a better position to defend next time? I'm not sure I follow.

    As for me, anyway, I just use the basic combat rules for crits these days - no defense, no table rolls, a 3 is maximum damage. It's just faster given the amount of them that show up in a high-powered game.

    1. I was in the process of editing this post and got interrupted. I was just sitting down to edit it when I saw your response. I don't like the "no defense on a critical hit" rule in most games. I have a few different ideas for what can replace it. I think the "no defense" rule in combination with my suggested free change of hit location would be much more powerful (though also more easily explained in terms of in-game metaphysics. I know how to describe a laser striking an eyeball is doing; I don't know how to describe a triple damage laser) than the damage increases.

      I also think I need to replace some of these results to give more functionality against non-human targets like robots and vehicles.

      On the double and triple damage: I don't see any way to reconcile the effect with what I consider damage to be (outside of quantum weirdness). I've seen it come up often enough that I can't explain these common extreme overpenetrations of armor as one-in-a-trillion shots, since they come up so often. I've had multiple sessions in a row where players lost armored characters to critical hits that pushed damage far outside the expected values.

      I also very strongly dislike the halved DR. There might be possible explanations for it on some objects, such as those with actual weak spots, but it seems insane to me that someone can shoot at a tank, battleship, or sealed TL11 space armor and sometimes happen to hit a spot that has half the protection guaranteed by its manufacturer. These weaknesses would be so apparent that anyone with a working eyeball would spot them. And if you assume there aren't any structural weaknesses in the armor, then how is the halved DR to be explained? If the effect is intended to represent hitting a weak spot or a gap in armor, then why not replace it with that option?

      I don't have the Acrobatics skill for the mental gymnastics necessary to explain these results. GURPS has a very strong anti-armor bias. That's fine to emulate Hollywood action films where armor doesn't work, but those aren't the games I'm trying to run, and the rules I'm proposing are intended to offer an alternative.

    2. If Critical Hits don't bypass defenses, then what do they do? 10,11 on your chart are "no additional effect." What's the basic effect of rolling a critical hit? That's a big change to GURPS, which is fine, but it's something that's worth stating right up front for your table - that it assumes that critical hits don't work as written, but do XYZ instead of ABC.

      Personally, I have no issue with description of 2x or 3x damage effects. I don't think of the damage range of a strike as being "minimal possible" and "maximum possible" bounded ranges, but "expected results outside of freak situations." If 3x damage comes up from the laser, it's from what it hit, now how hard it hit, and it's a cascade of injury results that normally wouldn't be expected to occur but due to unforseen (and for the attacker) fortunate coincidence, happened.

      Same with armor - and it's funny you should mention vehicles like tanks and battleships. They're what I had in mind when I mentioned shot traps. Armor on vehicles is no more uniform than it is on the human body - often less so. Previous damage, poor choices by the crew in equipment stashing, design flaws, design decisions (the glacis plate isn't uniform thickness on a tank, due to expectations about where it'll get hit at what angle), angle of attack, etc. all allow for impacts on a generally protected area against what will be less effective armor.

      I'd venture to say that armor that is uniform and provides manufacturer-stated protection in all cases from all angles is probably vanishingly rare.

      Do you allow chinks in armor to be targeted at all?

    3. Sorry if the reply sounds aggressive or argumentative - I'm not trying to be a jerk, just trying to be direct and clear!

    4. I've edited the post.

      I allow targeting the gaps in armors, as per Low-Tech. I don't know if the chinks rule can be salvaged for all rigid armors. I would have to go through them individually. What would a chink in the sealed combat armors from Ultra-Tech look like? A chink in a hoplite's shield? There are lots of things that provide DR that don't seem to have areas with half the DR on them. Maybe there are areas with less DR, but not half.