Playbook: Armour Damage Threshold

Long before I started this blog or joined the GURPS Discord, me and Jose of Blogging At Default, Taking Extra Time have regularly bounced ideas back and forth, discussed things and come up with some new rules to handle situations. One of the things we had both run into is that, especially at high TLs, weapons can't keep up with armour, especially not military-grade examples. Someone in a combat hardsuit is all but invincible to any handheld weapons of the same TL. This presents an annoying combat balance problem if other players aren't wearing the same armour, as anything that might seriously threaten the hardsuited character will simply turn the rest of the party into red mist.

So the solution we came up with was a system to gradually degrade the DR of armours, based on the attacks that are hitting it.

Reasoning and Intent
Armour damage threshold is intended as an optional rule. Fair warning, these rules require a lot of extra bookkeeping and can be quite complicated, so they won't be for everyone. There are two main reasons you may want to use it. Firstly, if you want harsh realism, forcing players to either repair or discard armour after it's been used. Secondly, if you're using the stock armour in Ultra-Tech. These armours have DR high enough to render most hand-held weapons of the same TL totally ineffective, so armour damage thresholds provide a way to wear down DR by attrition, allowing people wearing these armours to be threatened by lower-yield weapons without making armour totally pointless.

Armour Damage Threshold
All armour has a Damage Threshold (abbreviated DT) that depends on TL:

TL0-2: DT 6
TL3-5: DT 8
TL6-8: DT 10
TL9: DT 15
TL10: DT 20
TL11: DT 40
TL12: DT 60

Modifiers:
  ×0.5 if semi-ablative
  ×0.8 if Cheap quality (minimum -1)
  ×1.1 if Fine quality (minimum +1)
  ×1.2 if Very Fine quality (minimum +2)
  ×1.5 for heavy-duty armour (mail-and-plates, ballistic trauma plates, combat hardsuits, etc.)

When an attack hits armour, total the basic damage it dealt and multiply it by the attack's armour divisor. If the attack's damage type was corrosive, quadruple the final total. This is the attack's armour damage. Divide the armour damage by the armour's Damage Threshold, dropping decimals. The armour loses this many points of DR. If the attack penetrated the armour, it loses an extra point of DR for every 5 points of basic damage that penetrated, minimum 1 point.

For an armour that has split DR, take note of the DR multiplier. This is the number the lower DR is multiplied by to get the higher DR. In the case of nanoweave with DR 18/6*, 6×3 = 18, so the multiplier is 3. When the lower DR is damaged, reduce the higher DR by same number times the multiplier. When the higher DR is damaged, only reduce the lower DR if the DR damage was equal to the multiplier.

A piece of armour that has had its DR reduced is damaged. Damaged armour can be repaired with an Armoury (Body Armour) roll. Once an armour's DR has been reduced to 0, it is broken. It may not be repaired, only scavenged for raw materials.

Examples:
  TL4 medium plate (GURPS Low-Tech, p. 111) has DR 6, and is hit with a maul wielded by a ST 13 character. The maul deals 2d+4 cr, rolling a 11, so 5 points of damage penetrates. The attack's armour damage is 12. Dividing by the medium plate's DT 8, the result is 1, so the plate loses 1 point of DR, becoming DR 5. In addition, 5 points of damage penetrated: 5÷5 = 1, so the armour loses an additional 1 DR, becoming DR 4.
  A TL8 assault vest with trauma plates (GURPS High-Tech, p. 66) has DR 35 vs. piercing damage, and is hit with an AK-47. The AK deals 5d+1 pi, rolling a 17, so no damage penetrates. The attack's armour damage is 17. The trauma plates are both semi-ablative and heavy-duty, so their DT is unchanged. Dividing by the vest's DT 10, the result is 1, so the trauma plate loses 1 point of DR and becomes DR 34.
  TL10 nanoweave armour (GURPS Ultra-Tech, p. 172) has DR 18/6*, and is hit with a shot from a laser carbine. The laser deals 5d(2) burn, rolling an 18, so 12 points of basic damage penetrates (as the lower DR applies vs. burning damage). The armour damage of this attack is 18×2 = 36. Dividing by the nanoweave's DT 20, the result is 1, so the nanoweave loses 1 point of DR, becoming DR 15/5*. In addition, 12 points of damage penetrated; 12÷5 = 2.4, so the armour loses an additional 2 DR, becoming DR 9/3*.

Targeting Holes in Armour
Any attack that penetrates armour other than crushing will leave a hole in the surface. Attacks to holes in armour totally ignores the armour's DR! To target a hole in armour, the penalty depends on the damage type of the attack that penetrated:

Damage   Penalty
imp, pi++    -5
cut, pi+     -6
corr, burn, pi    -7
pi-       -8