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
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.