Armor as Dice – what and why?
The term “Armor as Dice” was coined somewhere on the GURPS forums to describe a method of treating armor using (oddly enough) dice instead of points of DR. It was a bit of parallel evolution – while I wasn’t the only one to come up with the phrase, I was using it in games as early as October 2004.
I expanded on the concept, which is pretty much what it says on the tin, in my short article “Armor Revisited,” from Pyramid #3/34: Alternate GURPS.
The way I do this in my games featuring guns is that I convert armor to dice of resistance, at 3.5 points per die. Armor gets the benefit of the doubt (that is, a remainder of 1.5 becomes 2) but you have to earn each full die. Meaning that 12pts of damage becomes 3d+1.5 = 3d+2 protection, while 13pts id 3d+3, rather than 4d-1, because that extra die can be very important.
To be very clear: you subtract armor dice from damage dice. You don’t roll until after you’ve done this, and that is an injury roll, which is then modified for bullet size, hit location wound modifiers, etc.
For flexible ballistic armor that lacks stuff like shear-thickening fluids in them, the bullet tends to either go right through with hardly any slowdown, or get totally stopped. For this type of armor, a special rule: if the attack average damage is more than the armor DR – higher dice of damage than armor DR dice – you subtract one point of damage per die of armor.
So a 5d bullet hits a 3d+2 (DR 12) flexible armor vest. Damage is not 2d-2 like it would be for hard armor, but 5d-3. Ouch. This is the way kevlar works in reality, and if you look up the NIJ threat levels as presented in GURPS Cops, and convert them to dice, you match reality very well.
This means if you overmatch a flexible vest by even just a little you will overwhelm it and do a lot ofdamage. If you have a 3d-1 bullet (average damage 9.5) vs a 2d+2 kevlar vest (average protection 9), it only gets 2pts of protection (2d is 2pts), and net damage is 3d-3.
Ties go to the armor; or if you can handle a touch of complexity, roll 1d-4 whenever the dice of damage are equal, but the numbers aren’t. So 3d-1 bullet vs a 3d vest becomes (3d-1) – 3d = (1d-4) -1 = 1d-5; roll a 5 and it’s a zero damage breach of the armor; roll a 6 and it’s one point of penetration; roll 1-4 and the armor stops it.
Great, so you should always do this?
Probably not. While this works quite well and improves verisimilitude for bullets (and actually helps speed of play in most instances), for hand-to-hand combat in fantasy realms it doesn’t work so well as described. Firstly, the strength and power of a hand-delivered blow can be quite variable, while the energy delivered by a gunshot is usually within a few percent of the average for each time (at least out of the muzzle).
Hand to Hand
Not only is the damage from a melee blow pretty variable, armor can be variably thick. Not only “can be,” but at 
least for late-stage plate, nearly always was. That could be leveraged in interesting ways (roll 2d+1 for damage, but armor is 3+1d6!) if you want to do such a thing, but now you’re getting into complexity for the sake of it.
Also, While DR 7 –> 2d makes good sense, values from DR 1 (about 365 microns of RHA steel!) to DR 6 are probably best left as DR values rather than dice.
Ranged Weapons Revisited
represented by a called shot (“I shoot his armpit!” or “I aim for the side panels of the vest”) where DR is halved at the usual huge penalty (something like -8) or a critical hit that has the same result.And if you get variable penetration and variable thickness armor, well, you might as well roll damage anyway, right?
Keep it Simple, Keep it Safe
Armor as Dice was created to reduce the penetration variability of weapons – usually guns – that should be 100% stopped by a given piece of armor, but punch through because of the potential 70% higher damage that can be rolled compared to average penetration. Since GURPS armor values are in fact set at average penetration (DR 70 = 2d = 1″ RHA steel), this provides too much oomph to projectiles.
So if you’re going to do this, just subtract armor dice from damage dice, and roll the remainder as injury.
Split the Difference
Parting Shot
I periodically mention that I’m going to start a game here at some point, likely something like a GURPS: X-Com or Monster Hunters style game that will basically be Dungeon Fantasy but with guns. Or maybe swords and guns. Or sword guns.
"only use dice for DR7+, while DR1-6 would just be . . . DR 1-6." – are there ever situations you'd run into with this where you'd get better protection from DR6 than you did from DR7, converted to 2d, using your rule for flexible/kevlar armor?
That's a good point, and you would basically look at DR1-3 either blocking all the meager damage, or nothing. DR 4-6 would block it all (say, a .22 LR) or lose one point.
Also, your bolded text above makes it clear that you only roll the difference in dice – why is that? Why not roll both the attack and the armor, aside from an aversion to adding another contested roll?
The entire issue, I think, with variable damage (and even worse with variable damage AND armor) is that the overall variability in penetration is much, much less than such randomization would indicate.
The great example is to consider a .45 ACP against a NIJ Level II vest . .. actually, this is worthy of it's own post. Stay tuned.
Wouldn't it be simpler to rescale the firearms (ex 5d-2->1d+12)? That way there is not a different DR model between combat modes.
That's a pretty good way of doing it, and one touched on in this post and the last one. I'm actually going to amend this with a table of reduced variability, though one of the issues will be how to get low enough variability in small calibers. It's straight-forward, for example to recast 6d into 18+1d. Less so to recast 1d or 2d into something else. Still, I think it's doable, if you're willing to experiment a bit.
I’ve done some digging and have found, using your rules, what I consider the dr of the different NIJ levels should be
NIJ I = 7dr=2d
NIJ IIA = 9dr=2d+2
NIJ II = 11dr=3d+1 (actually 3d+0,5)
NIJ IIIA = 13dr=3d+3 (actually 3d+2,5)
NIJ III = 25dr=7d
NIJ IIII = 25dr=7d with hardened and either semi-ablative or some more exotic homebrewed rule like I have.
There are some interesting things with these numbers that i want to talk about. First, with these numbers a level I vest can stop for example a 9*19mm bullet (2d+2 damage), something it is not rated for. However from what I found out this is quite accurate, as Vests often stop bullets that are rated higher than what they are supposed to combat. The problem is usually the deformation caused by the bullet forcing the vest backwards. If we use your 1d-4 rule we can quite handsomely simulate backface deformation of bullets, by ruling that the damage received is crushing damage and that the vest isn’t penetrated. That means that a level 1 vest could stop a 9*19mm bullet, but the potential crushing damage could still be 4, not a laughing matter at all.
What do you think?
It appears that GURPS Thaumatology: Sorcery page 24 borrowed this for Reverse Missiles, also setting a precedent that dice DR (or at least, dice DR with Force Field) can stop Afflictions outright rather than merely giving a bonus to your HT roll against them, in the same ratio as an Innate Attack does when used as a Power Parry, but without needing to roll a parry!
Of course, you’d need DR 4 (1d) to bounce back Affliction 1, DR 7 (2d) to bounce back Affliction 2, and so on. Seems like a great counter to the “I have 100 Will and I am using Affliction with Malediction against you, you will never win this Quick Contest” approach.