The penetration of weapons is usually quite constant. A bullet might go through 5 mm of rolled homogenous steel, for example, not anywhere from 2 to 12 mm. However, the severity of the wound can vary by quite a lot - a difference of a centimeter in the wound channel can make the difference between a wound that severs a vital structure and one that doesn't affect anything particularly important.
To reflect this, armor can directly subtract from the dice of damage done. Rather than DR 12, that armor will now be 3d+3. This will subtract 3d from the damage before the damage is rolled, and then an additional flat 3 points. If you shot someone wearing this armor with a 5d+1 attack, the armor wearer would take (5d-3d)+(1-3)=2d-2 damage.
Weapons with armor divisors will often end up with "fractional dice" or armor. For example, if 5d+4 armor was hit by an attack with an armor divisor of (2), the armor would protect with 2d+2+1/2 d. But what is 1/2 d? The basic rule will be to drop all die fractions of less than 1/4, treat die fractions of 1/4 or more but less than 1/2 as +1, and die fractions of 1/2 or more as +2. The chart below shows all fractional dice that should arise in gameplay:
|2||1/2 d = +2|
|3||1/3 d = +1||2/3 d = +2|
|5||1/5 d = 0||2/5 d = +1||3/5 d = +2||4/5 d = +2|
|10||1/10 d = 0||2/10 d = 0||3/10 d = +1||4/10 d = +1||5/10 d = +2||6/10 d = +2||7/10 d = +2||8/10 d = +2||9/10 d = +2|
If all that gets through is a constant damage add before rolling, then +1 causes 1d/2, and +2 causes 1d/2+1. If any dice of damage get through, just add the +1 or +2 damage normally.
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