This is a spin-off from the current Tweak of the Week thread, Armour as Dice.
This is meant to be a relatively simple way to determine how a given piece of armor protects a target, without using the same degree of variability inherent in damage dice. The idea is that a given chunk of armor is going to give very consistent protection against a single threat, but that the actual angle of the impact is going to skew things a bit.
The DR values in published GURPS books* are based on an optimal impact at 0 degrees. These values are used as the nominal DR - attacks will often hit at an angle, functionally increasing this value. There are two major effects that cause this increase. The first is that hitting at a poor angle actually increases the amount of material the impactor must contend with. The second is that, with rigid armor made of materials that are of comparable or greater hardness to the impactor, hitting at an angle can also deflect the bullet for an even greater effective increase in DR. I assume the two effects are ultimately multiplicative, although I could be mistaken here.
*Many LT values apparently have a fudge factor of +1 DR or so to account for deflection; it probably won't be horrible to just ignore this fact.
The system itself is relatively simple. For soft or flexible armor, roll 1d-2 and divide by 2, rounding down. For rigid, hard armor, simply roll 1d-2. A result of 0 or less means the impact was more-or-less dead-center, for full nominal DR. Beyond this, every +1 to the result is +1 SSR (x1.5, x2, x3, etc) to DR. Following this "Armor Roll," resolve the hit they same way you normally would. If you'd like potential further complications, read on.
Chinks, Gaps, and Glances: On a natural roll of 1, roll again. If you roll another 1, you hit an Armor Gap (if there is one in the impacted area) or a Chink for -2 SSR to DR (x0.5 DR). If you roll a 2, you hit a lesser Chink for only -1 SSR to DR (x0.7 DR). A 3 or more means you simply deal with normal DR. Flexible armor only has lesser Chinks.
Similarly, on a natural roll of 6, roll again. If you roll another 6, the hit glances off and target enjoys a further +2 SSR to DR, while a 5 means a further +1 SSR to DR instead. A 4 or less means dealing with x5 (hard) or x2 (soft) DR, as normal. Soft armor can only get up to +1 SSR for a glance.
Expertly or Masterfully Tailored armor usually gives a penalty to purposefully hit Chinks and Gaps. Optionally, if a character randomly hits a -2 SSR Chink or a Gap on such armor, require a final roll. For Expertly Tailored armor, a roll of 5 or 6 on this final roll means you instead hit a -1 SSR Chink. For Masterfully Tailored armor, a roll of 3 or greater means you instead hit a -1 SSR Chink.
Barrier Blind: Some projectiles or other impactors may be designed to partially ignore the effects of angle of impact. This is also true of pretty much any hypervelocity impactor (including shaped jets and the like). For such projectiles, anything they are "blind" to should be counted as soft, even if it's otherwise far harder than the projectile - a hypervelocity pillow would treat diamond as soft!
Sloped Armor: Some armor is actually shaped to present a more sloped face to potential impactors. This makes the armor less mass-efficient against true face-on impacts, but improves the chance of the impact being at a poor angle. Subtle, well-engineered sloping is called Fluting and works equally well against melee and ranged threats; more extreme sloping is easier to design but can be overcome fairly easily in melee, making it only usable against ranged threats. Fluting is +4 CF, +10% to weight, and +1 to the Armor Roll. More extreme sloping is +25% to weight and cost per +1 to the Armor Roll. Fluting cannot be overcome, while with extreme sloping you can ignore the bonus by striking at a more optimal angle. For melee this is automatic (but may justify a +1 to defense due to predictability), while ranged requires you to actually be firing from a good location (GM's call) or using some sort of arcing trajectory.
A Matter of Skill: Optionally, a character's skill can make them more or less likely to hit at a poor angle. For Harsh Realism, having combat skill below DX means a +1 to the Armor Roll, while working off of default means a +2 to the Armor Roll. On the cinematic side, a -4 or so to skill may be enough to justify a -1 to the Armor Roll (for soft armor, apply this -1 before halving), and ending up with a -1 or less means you have a chance to hit Chinks/Gaps (a natural 6 still risks a Glance, regardless of skill).
On the other side, an inhumanly competent character might be able to purposefully angle his or her armor to better intercept an incoming attack. Each -2 or so to Active Defense gives a +1 to the Armor Roll (again, for soft armor, apply this +1 before halving). A final result of 4 or more means a chance of a Glance (a natural 1 still risks hitting a Chink or Gap). A GM may require the ETS or Precognitive Parry to pull this off against firearms and the like.
This is meant to be a relatively simple way to determine how a given piece of armor protects a target, without using the same degree of variability inherent in damage dice. The idea is that a given chunk of armor is going to give very consistent protection against a single threat, but that the actual angle of the impact is going to skew things a bit.
The DR values in published GURPS books* are based on an optimal impact at 0 degrees. These values are used as the nominal DR - attacks will often hit at an angle, functionally increasing this value. There are two major effects that cause this increase. The first is that hitting at a poor angle actually increases the amount of material the impactor must contend with. The second is that, with rigid armor made of materials that are of comparable or greater hardness to the impactor, hitting at an angle can also deflect the bullet for an even greater effective increase in DR. I assume the two effects are ultimately multiplicative, although I could be mistaken here.
*Many LT values apparently have a fudge factor of +1 DR or so to account for deflection; it probably won't be horrible to just ignore this fact.
The system itself is relatively simple. For soft or flexible armor, roll 1d-2 and divide by 2, rounding down. For rigid, hard armor, simply roll 1d-2. A result of 0 or less means the impact was more-or-less dead-center, for full nominal DR. Beyond this, every +1 to the result is +1 SSR (x1.5, x2, x3, etc) to DR. Following this "Armor Roll," resolve the hit they same way you normally would. If you'd like potential further complications, read on.
Chinks, Gaps, and Glances: On a natural roll of 1, roll again. If you roll another 1, you hit an Armor Gap (if there is one in the impacted area) or a Chink for -2 SSR to DR (x0.5 DR). If you roll a 2, you hit a lesser Chink for only -1 SSR to DR (x0.7 DR). A 3 or more means you simply deal with normal DR. Flexible armor only has lesser Chinks.
Similarly, on a natural roll of 6, roll again. If you roll another 6, the hit glances off and target enjoys a further +2 SSR to DR, while a 5 means a further +1 SSR to DR instead. A 4 or less means dealing with x5 (hard) or x2 (soft) DR, as normal. Soft armor can only get up to +1 SSR for a glance.
Expertly or Masterfully Tailored armor usually gives a penalty to purposefully hit Chinks and Gaps. Optionally, if a character randomly hits a -2 SSR Chink or a Gap on such armor, require a final roll. For Expertly Tailored armor, a roll of 5 or 6 on this final roll means you instead hit a -1 SSR Chink. For Masterfully Tailored armor, a roll of 3 or greater means you instead hit a -1 SSR Chink.
Barrier Blind: Some projectiles or other impactors may be designed to partially ignore the effects of angle of impact. This is also true of pretty much any hypervelocity impactor (including shaped jets and the like). For such projectiles, anything they are "blind" to should be counted as soft, even if it's otherwise far harder than the projectile - a hypervelocity pillow would treat diamond as soft!
Sloped Armor: Some armor is actually shaped to present a more sloped face to potential impactors. This makes the armor less mass-efficient against true face-on impacts, but improves the chance of the impact being at a poor angle. Subtle, well-engineered sloping is called Fluting and works equally well against melee and ranged threats; more extreme sloping is easier to design but can be overcome fairly easily in melee, making it only usable against ranged threats. Fluting is +4 CF, +10% to weight, and +1 to the Armor Roll. More extreme sloping is +25% to weight and cost per +1 to the Armor Roll. Fluting cannot be overcome, while with extreme sloping you can ignore the bonus by striking at a more optimal angle. For melee this is automatic (but may justify a +1 to defense due to predictability), while ranged requires you to actually be firing from a good location (GM's call) or using some sort of arcing trajectory.
A Matter of Skill: Optionally, a character's skill can make them more or less likely to hit at a poor angle. For Harsh Realism, having combat skill below DX means a +1 to the Armor Roll, while working off of default means a +2 to the Armor Roll. On the cinematic side, a -4 or so to skill may be enough to justify a -1 to the Armor Roll (for soft armor, apply this -1 before halving), and ending up with a -1 or less means you have a chance to hit Chinks/Gaps (a natural 6 still risks a Glance, regardless of skill).
On the other side, an inhumanly competent character might be able to purposefully angle his or her armor to better intercept an incoming attack. Each -2 or so to Active Defense gives a +1 to the Armor Roll (again, for soft armor, apply this +1 before halving). A final result of 4 or more means a chance of a Glance (a natural 1 still risks hitting a Chink or Gap). A GM may require the ETS or Precognitive Parry to pull this off against firearms and the like.
Angle of Impact and DR
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