Ballistic Ratings and the WTF of Armor as Dice

Yesterday’s post on Armor as Dice generated more commentary than any content-related post I’ve had in a while. So booyah, that’s good. Lively discussion and all that.

However, +Jason Packer asked a question that echoed (and contrasted with) another poster’s comment about ensuring that you just subtract armor from damage and, if this is greater than zero, rolling the rest as injury.

Why not roll both?

The answer doesn’t lie in the realm of complexity or avoidance of such. It lies in the realm of observed behavior of real-world stuff.

Ballistic Protection and You

Let’s start with page 2 of Ballistic Resistance of Body Armor: NIJ Standard-0101.06, where the definition of a Level IIA vest is discussed:

2.1 Type IIA (9 mm; .40 S&W) 

Type IIA armor that is new and unworn shall be tested with 9 mm Full Metal Jacketed
Round Nose (FMJ RN) bullets with a specified mass of 8.0 g (124 gr) and a velocity of 373 m/s ± 9.1 m/s (1225 ft/s ± 30 ft/s) and with .40 S&W Full Metal Jacketed (FMJ) bullets with a specified mass of 11.7 g (180 gr) and a velocity of 352 m/s ± 9.1 m/s (1155 ft/s ± 30 ft/s). 

Type IIA armor that has been conditioned shall be tested with 9 mm FMJ RN bullets with a specified mass of 8.0 g (124 gr) and a velocity of 355 m/s ± 9.1 m/s (1165 ft/s ± 30 ft/s) and with .40 S&W FMJ bullets with a specified mass of 11.7 g (180 gr) and a velocity of 325 m/s ± 9.1 m/s (1065 ft/s ± 30 ft/s).

OK, from a GURPS standpoint, we’re looking at 124gr 9mm ammo at 373m/s, and 180gr .40S&W at 352 m/s. If you use my bullet model, that’s 10.1 points of penetration for the .40S&W, or 9.1 points for the 9mm – surprisingly to me, the .40S&W is the greater threat.

So based on these criteria, a threat level IIA vest should stop that average damage (that’s how GURPS DR is defined), so let’s call it DR 10. All of this is for a new vest, but the upshot of the “conditioned” vest standards is about a point less, or about DR 9.

Note that threat standards have gone up over time. I believe a Level IIA vest used to be rated more along the lines of .22 LR and lower velocity .38 special, maybe .45ACP . . . but NIJ revised their standards in July 2008. The IIA used to be DR 8 or thereabouts. I’d have to go back on old Forum posts for that one, or just check GURPS Cops, by Lisa Steele.

Now, a .45 ACP has but 450J or so, compared to the very hot 725J (hotter than a lot of standard 10mm Auto bullets!) contained in the .40S&W spec. A Level IIA vest will stop a .45ACP pretty much always under the test conditions.

But the variability in penetration if you roll damage vs DR 10 is 2-12, so there’s a small but real chance of a .45 ACP defeating this vest. If you take a more reasonable .40S&W at 320m/s (1050fps or just shy of 600J) for 9.1 points of damage, the vest should always defeat it, but 2d+2 vs DR 10 has a reasonable chance of overmatching the vest. A weapon down to 2d-1 is still a legit threat, and it really shouldn’t be.

Thus, the “Armor as Dice” concept, which applies the average damage of a bullet, 2d for a .45 ACP, 2d+1 or 2d+2 pi+ for .40S&W depending on load, 2d+2 pi for 9mm NATO standard to the average DR (in dice) of the bullet. You have to overmatch the armor before a penetration occurs.

You could get similar effects by taking the bullet average damage (7, 9, and 9 respectively) and looking at DR 10 and saying “nope.”

Missing the vest and hitting the armor is usually handled by critical hits (one option is halving DR) or targeting chinks.

In fact, considering critical hits, it might be a good idea to rewrite the table if using Armor as Dice – or at least re-interpret it such that for firearms, any “max damage” results have an armor-reducing effect instead. “Max Normal Damage” and “Double Damage” results should be rescoped for this case as “half DR.” “Triple Normal Damage” would be easy as “1/3 DR or Armor only provided one point of DR per die,” which are mildly equivalent.

Parting Shot

The overall point of armor as dice – originally – was to ensure that the variability of a normal 1d6 or 2d6 distribution didn’t overwhelm plausible verisimilitude by providing the propensity for a teeny bullet like a .380 ACP from overmatching armor or a vest that it would simply never do in real life. The .45ACP, which is a fine man-stopper but being huge and slow is a poor penetrator, is a more credible example of the type of round that is stopped fairly routinely by lower protective armor.

The side effect of this, in play for me, was much simpler calculation of “threat/no threat” decisions in the black ops game I played a while back, where I could pretty much instantly determine how much injury a victim would take with very quick math. 
But the overall question, “why not roll both?” goes in the opposite direction of what expressing dice of DR is supposed to accomplish.
How about Partial Variability

Now, if you want more complexity, either because you do much of this stuff with a computer or you just groove on it, I’d probably look to one of the optional rules in Armor Revisited to fix this. 6d damage would be expressed as 18+1d instead of 6d. DR 18 might instead be something like 10+2d. You could roll both and see what goes through (8-1d injury).
I’m not sure I’d do it that way without a computerized game aid, but it’s certainly doable.

8 thoughts on “Ballistic Ratings and the WTF of Armor as Dice

  1. Yay for more BALLISTIC posts on GB! You'll have to let me know if you do an Xcom game open to the public. I'd love to see how you would handle it.

    The autofire post was also great. I think you should create a new section of your blog for collecting gun-related posts–maybe The Firing Range–to go along with Mêlée Academy!

  2. You have indicated that Kevlar is very much all or nothing, and should probably have some special rules around how to deal with bullets that actually exceed tolerances. Do trauma plates mitigate this disadvantage, or would a round that penetrates both vest and plates still do close to full damage?

    1. Off the cuff, I'd pick between these two:

      * Just subtract the whole damage, since the rigidity of the outer plate deforms the bullet and reduces the penetration effectiveness of what's left. This has simplicity and some basis in reality on it's side.

      * Subtracf the hard DR first, look at the remaining energy and compare it to the undervest. The bullet should probably lower armor divisor one step, so a (1) becomes a (0.5) or a (2) becomes a (1). This is more complex at the table, and would need some research for reality check.

  3. A variant I've considered is eliminating damage dice totally — instead, damage (penetration) is fixed, and you roll a die to determine the wounding multiplier. The 6d attack would just have penetration 21, so against DR 18 it would have a base of 3 — which would then be adjusted for wounding, likely with a result in the 1-6 range (depending on what scheme you use). That results in higher randomness for high power bullets; under a scheme where 3 becomes 1-6, a hit on an unarmored target would have a decent chance of doing as little as 7 or as much as 42.

    1. Any chance your system could be tweaked to *always* allow minimum damage to reach 1? Minimum damage thresholds that don't allow for the full range of possible results are a bete noire of mine…

    2. Actually, there are a couple of ways.

      You can use one of the the graze rules.

      You can also recast all your dice, so 6d (6-36) becomes 8d-7 (1-39).

      You can even recast and set up multipliers, so 6d becomes (1d*8)-7, so the lower numbers aren't implausibly rare.

      All of these have annoying failure points though.

  4. Just realized this while I was trolling around on fora for other game systems (I know, I'm such a tramp) – Hero System added this mechanic to their latest edition. In addition to Physical Defense and Energy Defense (DR, split by attack type) and Damage Reduction (a flat percentage reduction in damage), they added Damage Negation, bought in damage classes (which translate into dice at different ratios depending on the type of attack).

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