Grand Unified Smackdown Theory (Part 2)

I speculate that GURPS (and maybe The Fantasy Trip; I wasn’t around for that one) started with a fairly generic view of damage that incorporated penetration, blunt trauma, injury, and a certain amount of gamist fun. GURPS High Tech introduced the dependence on the square root of kinetic energy as a “damage” scale by giving the general rule of 20d = DR 70 = penetration of one inch of armor steel. I’m not sure from whence that one came, but I suspect David Pulver provided it to Mike Hurst

All of a sudden, you had two different progressions for damage, and some projectiles (bows) were on the ST-based scale, while the more-easily-quantified set (guns) were on another.

Sometime around the year 2000, I started fiddling with Excel’s solver. I thought that if I could find enough penetration data, I should be able to take a priori data about projectiles (specifically bullets) and turn that into GURPS stats. The result got me a mention on TV Tropes (look for Arbitrary Gun Power), and was published in 2002. But it worked fairly well, and got me noticed by a few other gamers

Anyway, THAT little adventure got me to notice that while firearms matched penetration of armor with known and quantifiable data – in this case the mass, velocity, and caliber of the bullet (from which you derive kinetic energy and momentum – that and cross-section are all you need to derive basic damage/penetration and a wound channel size modifier), weapons on the melee scale (things that you look up as “thr+2” or “swing+3” on the Damage Table on p. B16 of GURPS Basic Set, Fourth Edition) got pretty high penetration numbers pretty fast. Maybe too fast.

I once again scored a mention in TV Tropes by nearly breaking my brain (and Steven Marsh‘s sanity) figuring out how to do for bows and arrows what I did for guns. It was phenomenally more difficult. 

OK, so I’ve covered arrows and bullets. What about sling stones? Swords? Maces? Polearms? Pointed sticks? 

Um . . .

My previous post was an attempt to show how you could at least get the scaling of melee weapons right, where every doubling of ST also doubles penetration.

What you say?! GURPS ST measures applied force, and it goes as your ST score squared (Basic Lift is measured in pounds of force). Energy is force times distance, and if distance is more or less related to how far you can move your body (push-pull-twist-turn!), then doubling force should double deliverable energy, quadruple force is quadruple energy, etc. Then when you put this on the same sqrt(KE) progression as guns, you can arrive at a nice progression where ST 10 is about 1d, ST 20 is 2d, ST 40 is 4d, etc.

Hey, that’s damn playable. Even for me.

Well, that’s penetration, great. But injury? GURPS as-is is probably more right than wrong here when it comes to squishing meatbags. People are notoriously fragile (and strangely and unpredictably robust as well). 

So then you get into wounds, penetrating trauma, blunt trauma, and other things.

Fortunately, GURPS already has the tools required to handle this.

It has a basic damage number that should be used to represent penetration.

It has damage type and damage size modifiers that I’d personally apply a bit differently (maybe that’ll be Part 3), but basically are a “wound channel severity” modifier. 

It has armor divisors to represent something that penetrates better (or worse) than the raw energy might otherwise indicate.

With those things in mind, the only thing that’s really missing is a better way to represent blunt trauma and other non-penetrating injury.

Were I to start from scratch, I’d try and figure out:

Penetration rating, probably mainly based on energy. This would be GURPS’ “basic damage,” but possibly on a new scale. I like the “ST equivalent” number from the last post, since many weapons just act as lever arms, multiplying force. Multipliers are also nice because they allow the quite-simple “add +1 per die” type math, which is even easy in play.

Blunt Trauma rating, probably based on momentum and impact force. I think this is mostly what is done for hand weapons now, implicitly.

Armor Penetration modifier based on shape, hardness, or other factors. This is the source of my assertion that impaling weapons (a damage type I loathe) should probably get a (2) armor divisor, due to all their force being concentrated on a tiny, hard point.

Wound/Injury modifier based on the size and depth of damage. If I were really good, you’d incorporate some sort of “you have to achieve so much penetration before you can get to a Vitals type location.” That would help scale certain effects where you really NEED to go deep (Giganotosaurus hunting! Or a Frumious Bandersnatch)

I’d also have blunt trauma (including punches) be resisted by a HT roll, possibly influenced by skill. During the writing of The Last Gasp, one of my helpers ran a boxing match, and we decided that a HT roll to avoid actual HP of damage (but preserving the AP loss damage causes) from punches would make a LOT of sense, since even with boxing gloves, a typical 15-round boxing match as played in GURPS would kill both fighters. Extending this to all blunt trauma wouldn’t be too much of a leap, and if there were a penalty to HT rolls based on the rolled damage or something, that would keep maces and staffs scary. Alternately, padded hands might do the trick for bonuses to HT. People CAN get killed in fights, after all. They just usually don’t.

None of this is to say “GURPS is broken, wah!” It’s the best playable simulation out there that you can nod your head and say “yep, I get that” for most things. But when the tool of choice for penetrating DR 8 plate armor is “give me a sword” rather than “give me a spiked warhammer” (this one, not that one) or “grab me my gun,” then some head scratching might be in order!

The key in all this is to keep it fun and playable. All this crap needs to be subsumed into a table and calculated but once, during character generation or on obtaining the item the first time, and then never again. Even as ugly as The Deadly Spring was, the math is all in a spreadsheet (generously provided by Steven along with the issue of the magazine!) done up front.

9 thoughts on “Grand Unified Smackdown Theory (Part 2)

  1. I have been following your articles and posts, and I find this subject to be quite interesting. However, I would not throw the baby with the bathwater. From a gamist point of view I like the 8 ST->+2d sw/+1d thr. It makes every point of St seem to provide a significant benefit, damage-wise. My chosen house rules (in development) is as follows (heavily inspired by Bruno's comments):

    Double HP, so you get twice as much HP per point of ST. Standalone HP costs 1 per point.
    Double DR for gear only.
    Make the ST damage table not slow down after ST 27, as it currently does. It keeps the 8 ST->+2d sw/+1d thr progression indefinitely.
    Double firearm damage.
    Give impaling weapons of at least Good quality (or maybe Fine quality) AD(2). Such weapons can additionally be given Fine (Armor Piercing) for +4 C, adding one level of Armor Divisor.

  2. That's not bad, though the problem still remains that the rate of scaling is wrong. The good news is that at least your (Bruno's) method pushes off the "WTF?" point to a more superhuman level of ST.

    Still, ST 14 is 2d swing, making someone who's emphatically in the human level of lifting power have the penetration capability of a .45 ACP pistol.

    The reason I scale it the way I do is because it's super easy to figure damage from ST. ST 1,254 is 125.4 dice.

    I feel there must be a good way to use a d10 for the decimals, though. 🙂

  3. Regarding the inches of steel:

    While writing VEHICLES 1st edition I analyzed all weapons in HIGH TECH that I had data for. The best data came from rocket launcher HEAT rounds as that was very precisely stated in source materials. This led to me to calculate that Mike had been using DR 70 = inch of steel. The numbers for heavy machine guns also matched fairly well. I wrote to Mike Hurst and asked if DR 70 = 1 inch RHA was his intent for all weapons. He said he didn't remember for sure, but it seemed to match, so I should treat it as such.

    The concept of armor divisors had been around in GURPS for a while (e.g., in x-ray lasers) but Mike and I worked jointly to codify the use of the (#) symbology for them and spread it through GURPS.

    1. Yeah, I was able to reproduce that as well, though it helped that the DR 70 = 1 inch of steel had already been established.

      My biggest "fitting issue" was the limited number of data points I had for medium-caliber weapons at the time (I'd not found the Tank Protection Levels website: and so I had data out to .50 caliber rifles . . . and then a bunch of data for 406mm battleship guns! So depending how I normalized, I could get odd results at either the smallarms or battleship end. I chose battleship for the oddities.

    2. That's interesting!

      What really drove me to distraction at one point was finding penetration figures for age of sail cannon…

      I did most of the early Vehicles research out of the Royal Military College library in Kingston (and its public library).

      Those were pre-laptop days, and almost all the books were in reference sections and many too big to copy so, I just stayed in all day filling notebooks with scrawls on system weights, penetrations, armor data, and other figures, then later transcribing them into my computer in Word Perfect.

  4. I agree the scaling you propose looks good for penetration, though I suspect it would be a bit too much for a retrofet.

    One think that is neglected a lot is knockback.

    It has bothered me for many years that firearms did not scale with fists for the purpose of knockback. I've sometimes speculated on an ideal where raw damage was *momentum* (well, probably square or cube root of it, as it's being compared to ST or HP) with the penetration layered atop that, rather than the other way around.

    E.g, a pistol bullet at 1d+1 (2), rifle at 1d+3 (4), and a strong punch at 2d (0.25). Set knockback at equal to target's base HP. (Obviously in some cases it represents losing balance.)

  5. Very interesting. If you look at my guns article from 2002 (wayback machine!), one of the things I did look at was how wounding modifier scaled up from penetration. If K is the Kinetic Energy, penetration went as sqrt(K).

    I found that wounding did pretty well with a term based on momentum times cross-section (call it P, and ignore the cross section for a moment), but that didn't work as sensibly as I'd like. However, when you took the geometric average of the two sqrt(sqrt(K) * P) you got sensible numbers for injury.

    I took this to mean that injury was enhanced by penetration, but more force and cross section was better. Thus, a shallow big wound was less effective than a deep big wound.

    The sqrt( sqrt(K) x P) term is INJURY. To make it into a multiplier, divide both sides by sqrt(K). so the wound channel mod is sqrt(P/sqrt(K)). That can be manipulated to give a 9mm a modifier of 1.0, which gave .45ACP exactly 1.5, just like regular GURPS!

    I think that the reason I like penetration first is that armor is the first thing you encounter as a general case (even if armor is missing).

    Your comment about knockback, and by extension knockdown/unbalance and blunt trauma is right on, I think. Your comment about the momentum term translating directly into these things is very appropriate (it's like you wrote the book or something), and would be a nice way to ensure that Ogres swinging trees turn adventurers into ballistic objects themselves!

  6. I'll have to look at your average momentum and cross section more closely. The results you suggest re. wounds seem intuitively correct.

    My main reason for going with momentum first is that it makes things slightly simpler for super games, overbearing and some collisions, but yes, my momentum-based values did not really cover injury.

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