An exploding damage die for GURPS?

Some of the recent threads and comments about armor as dice have led me to think about alternate ways to get what I want out of Armor as Dice – less variable penetration so that if you armor rated for X (and GURPS defines X as 3.5 points per die for both penetration and resistance), and a bullet hits you with basically less energy than X, it won’t go through.

Some of these distinctions don’t seem like much, or important. And to a certain extent, they’re very much not. If you have (say) a DR 8 bullet resistant vest, in theory it should be proof against a .45 ACP (2d pi+) but not a 9mm (2d+2 pi).

Turning to AnyDice (and we’ll be doing that a lot this post) we see that the .45ACP will punch through DR 8 with 1 point or more of damage potential remaining just over 27% of the time in round numbers. The 9mm, which should always go through, will go through 58% of the time.

This is very easy to rationalize. Poor angles, uncertain coverage, and other variables make armor less certain. The tendency to treat an armored vest as if it fully covers the entire torso (a legit simplification) makes the push to make every thing just work out neatly less mandatory.

Still, players buy armor to buy protection, and some armors really are that good. The “solution” tends to be “if you want full protection in a gamist manner, buy DR6 per die of protection you want.” Thus, if you want to be fully protected vs. that 2d or 2d+2 bullet, buy DR 12 and DR 14 respectively, which will increase the weight of that armor piece by about 70%.

OK, fine, but I am going to press forward and solve the non-problem anyway, because though there are many instances where you can rationalize the roll, there are others where you can’t.

Armor as Points, Penetration as Points


Armor as Dice has gotten plenty of love elsewhere. But there’s another method discussed in Armor Revisited (Pyr #3/34), which inverts the method: instead of listing Armor as dice and keeping the damage as dice, express the penetration as a fixed number, and go with variable injury.

So instead of 2d, a .45ACP handgun would have a pentration rating of 7, a 9mm would be 9, a 5.56x45mm might have a 17, and a .50 BMG would be something like 40-46 depending on the barrel.

So the effect is the same. Compare Penetration to DR, and if Pen is higher, it goes through. This also allows either using HP or “Mass-based” HP as a blow-through threshold as a number read right off a character sheet, precalculated before play starts for the mass-based number.

So, for a straight-up, GURPS standard application, subtract DR from PEN, and then roll injury dice. Roll 1d for every 3.5 points of penetration that get through would be the most straight-forward conversion, though “divide by 3 or 4” would be easier in play. Remainders might just be adds. Actually, rolling 2d per 7 PEN, and converting so you always roll two dice might not be that bad.

So a notional M4 carbine might do 16 penetration, and impacting DR 10 would have 6 remaining penetration points. Injury results, and you roll 1d+2 (average 5.5) or 2d-1 (average 6).

A tank cannon that usually does 6dx20(2) would convert to 420(2), and faced with DR500 would be 420 PEN – 250 DR (thanks to the armor divisor), leaving 170 PEN left. Against machines, maybe you don’t roll, maybe you convert to 2dx24 or even 3dx17 if you want some randomness.

Explode it!


But what if you want even more random injury – because injury is far, far more variable than penetration?

I’ve recently been exploring the Savage Worlds system, which features exploding dice. Called an Ace in the game, if you roll the highest value on your die, you get to roll it again, and add it to the prior roll. Leading to the unusual circumstance that you can’t roll a 6 on an exploding d6. But nevermind that.

An exploding die is basically a geometric progression. It’s the average value of the die (3.5 for d6), multiplied by 1 + the probability that you get to roll again (1/6) + the probability you get to roll a third time, fourth time, etc.

In short:

We’re borrowing a mechanic from another game here (Savage Worlds, but other games have had exploding dice before SW), so one has to be careful. 

Still, it’s a better fit – isolated to effect rolls, and applied to something that’s highly variable, an injury roll – than the D&D Advantaged mechanic is to the GURPS space. 

The impact on actual damage rolled isn’t that high – a 20% boost in damage if you let the dice explode infinitely, but honestly if you let the die explode 3 times (providing a potential 4x damage multiplier at the high end) you’re already averaging the 4.2 that is the asymptote. Since that’s the same as a brain hit (x4), you might as well cap it at 3 extra rolls per die.