Friday, April 11, 2014

GURPS melee combat; cut vs. thrust

One of the things that has always bugged me about GURPS is the melee combat damage rules. Namely, that you're better off trying to slash through armor with a sword than stab through it. Everything I've ever seen says hitting with the point is better for penetration.

First, a video, for demonstration purposes of stabbing working better than slashing against armor. We'll be looking at knives in particular, because I said so and am presently too tired to make the words work to explain why. Mostly because no matter the tech level, any character I build will probably have a knife, if not be an actual knife fighter.

By GURPS rules, all damage is the same, whether it's concentrated to a point or spread over an edge.

Now, to analyze the video. Those are very clearly all-out-attacks (strong, +2 damage), combined with reversed grip (+1 thrust damage). At ST 10 or higher, that +1 still isn't enough to put it on par in terms of damage with a swing, unless weapon damage modifies it.

Large Knives, without alternate grips, do Thrust impaling damage on a stab or Swing-2 on a cut, which puts the base damage output equal with either type for a ST 10 or ST 11 fighter. Beyond that, however, and the gap begins to widen substantially. At ST 13, a large knife will do 1d of damage thrusting, but 2d-3 of cutting damage; while both average rolls are the same, a good hit with a swing will do substantially more damage, 9 vs 6. The gap quickly widens from there, as swing damage improves at a much faster rate than thrust.

So, the rules say that against an armored opponent, you're better off hacking at them than using a pointy tip to try to get through. How do we fix this?

My first inclination involves reversed grip. On something like a knife, which can be swung much more handily than any larger weapon when reversed, change damage on impaling attacks to +1 Imp or Swing Imp, whichever is better, and implement Pick rules. In strong hands, this leads to knives being able to do a substantial amount of damage to overcome armor.

But it still has a problem; it leaves no distinction for getting through armor. I've seen testing on smaller knives where a dollar store T-shirt put over a chunk of cheap meat was enough to provide substantial protection. Un-clothed meat took drastically deeper slashes on a cut, but stabbing with the point and then cutting allowed for extremely deep wounds. The alternative is to give piercing weapons a small armor divisor, to represent their focusing of the energy. (.5) to (.75) would go a long way towards making points interact with DR in a more realistic manner.


  1. Basics... Sw-2 cut and Thr imp... ST 13 those are 2d-1 and 1d respectively.

    2d-1 is the same thing as 1d+3 (and I can never find the rule stating that in 4e; but it's there).

    Our lg knife does 1d imp and 1d+1 cut. Average roll of 1d is 3.5 (round down to 3) so on an unarmored opponent v torso the stab does 6 (3*2) and the slash does 6 (4*1.5)... Max damage is 12 (6*2) for imp and 10 (7*1.5 round down) for cut.

    Against limbs, impaling loses the damage bonus, cutting keeps it.

    Against brain and vitals, the bonus is replaced with the location's modifier (x4 and x3); and you can't slash to the vitals.

    Damage bonuses apply to what penetrates the armor.

    The knife shown in the video is very fine by GURPS standards, +2 to the raw damage for both cutting and impaling.

    This comes up for knives again and again... that blade in the Cold Steel vid is a _SMALL_ knife in GURPS terms. Most knives we think of as "small" are not weapons in GURPS terms, so the t-shirt should be giving ample protection.

    Sometimes I get the impression that you haven't been playing GURPS since it was Man to Man and gotten to ask these questions of the authors... :)

  2. Astonishing, it's almost like you'd be right about having been gaming a tad bit longer. It's entirely possible that you've been playing GURPS since when I was still learning how to use a pencil. Also, this was penned at about 2 in the morning, after getting up at 6 in the morning the previous day. Pretty much hit post and passed out immediately.

    I still think the problem is mostly with the interactions with armor, unarmored damage is mostly just fine. I do still kind of like both those options I came up with and my house rules for games will probably use both, give weapons that focus their impacts to a sharp tip a slight bonus vs. armor, and let reverse grip put a bit of extra oomph behind stabs.

    Also, looking at Low Tech/martial arts, Long Knife is at 15-23" overall length, assuming 4-5" of grip on most full size fixed blade knives I've seen, that puts it as around 10+ inches of blade. By extension, a large knife would have less than 10" of blade, overall length of less than 15". They don't specify the actual size range of Small or Large knives. Once folding knives come into play, with their extended grips to be equal/slightly longer than the blade, it becomes more prudent to work based on blade length. GURPS specifies tactical folding knives (typically 3.5-5" blade) as being Small Knives in High Tech, so we'll put a 6" as the changeover from Small to Large, or an overall length of around 10" or longer. Below 3.5" blade length (we'll assume that disguised push daggers listed have around this length of blade), it seems like it would be prudent to work with tiny knives as using some other skill (brawling/karate/etc), and treat as a bonus to punch damage similar to brass knuckles, similar to how I was trying to build spiked brass knuckles.

  3. Armor is why they came up with the crutches of damage modifiers instead of just letting them do more dice.

    And it IS a kludge, they admit it.

    Arrows appear to do X damage, but don't penetrate armor as well as that would indicate...

    I forgot to mention that anything Cold Steel demonstrates has to be treated as ideal conditions max damage roll if you try to figure out how it works in GURPS. Their demos are "on the range" as it were.

    And it's not just length that makes a knife a given kind. It's mass as well.

    When trying to make GURPS fit the real world the most important thing to remember is that π = 3. Trying to get more precision than that is wresting the pig.

    Digging up some ancient paperwork from my wife's collection (she used to work in an e-room as some kind of nurse). Slashing, in the real world, ends up with treatable, serious-but-not-life-threatening wounds. Stabbings are what kill people. GURPS weights them nearly the same for damage. Of course, those deaths from stabbings might be all thrusts to vitals which is denied to cutting damage.

    I was attempting to make a joke, not be snide when I mentioned how long I'd be at this.

    1. Yeah, no worries, I took it as a joke and attempted to joke back. Hooray failed jokes all around!

      Yeah, the numbers I've always heard were 6" of stabbing penetration anywhere on the torso will be almost guaranteed to cause death without proper treatment, or enough slashing to result in bleedout.

      And yeah, CS demos are definitely about as far into ideal as you can get. Fact remains that trying to stab armor should be way more effective. I think a marginal armor pen modifier for suitably pointy things is probably best. And I'm definitely letting reverse grip on knives use Swing damage and pick rules, they don't call it icepick grip for nothing.

      Mostly I just want to avoid "nobody will ever stab with a cut/thrust weapon because cut is always better because everything has some level of armor so more outright damage is always needed", beyond getting into such things as the "who hits first" rules in Martial Arts.

    2. A point and edge rapier is not a good slashing sword, but the option is there for situations that are not modeled where π is 3; like ending up corps à corps and slicing the neck on your withdrawl. GURPS simply doesn't have a rule for using the blade like that, so it might as well be a thrust only weapon despite the swinging stats.

      And some weapons really ARE better used one way against armor than the other. The back-spike on many polearms is the armor piercing side and the much wider cutting surface being for lighter armor or non-armored.

      I don't think that stabbing is more effective against actual armor despite the concentration of the forces into a smaller area. Mild steel sheet metal that we can buy today is not a good proxy for actual armor in surface hardness or typical contours (which make it more resistant to being punctured). And some weapons just flat suck at getting through any armor at all regardless of how they are used. Some suck until special circumstances allow them to be employed.

      Dan Howard is on the SJGames forums and he does real world testing with real weapons and armor.