The Platinum Palace
Armor as Damage Reduction
Using armor does not only keep you safe from attacks, they are by nature meant to diminish the amount of damage you take overall. An arrow will lose some of it’s power trying to go through the armor, a sword will clash against metal chains and a hammer will slam against plate (or in cases of chain, just fuck it, still reduces damage, the attack just happened to be slightly misplaced)
The rules for armor as Damage Reduction follows These Rules, but with the added modifications:
Enchantment bonus on an armor gives it’s full bonus to AC as usual. But due to the fact that magic armor are hardening the material used, making it sturdier, it also gives a DR bonus that stacks with the armor’s normal DR, as follows:
Enchantment bonus +1 & +2: +1 DR
Enchantment bonus +3 & +4: +2 DR
Enchantment bonus +5: +3 DR
Adamantine keeps it’s normal rules for how it’s armors gives a DR bonus, and it will stack with magic armor and the base armor, when calculating DR.
Mithral will also give a DR bonus, due to it being much stronger than steel. Mithral has one point DR lower than Adamantine for each armor category.
DR from armor stacks with any other type of DR, including other armor DR (If one would use multiple armors) or natural armor DR, as well as DR from special abilities, spells and other items giving a specific DR type. (unless it would make logical sense that it would not, ask your DM)
On a critical hit, an armor only offers half DR against the attack. If the weapon that dealt the crit was a piercing weapon, then the armor offers no DR (apart from that of enchantment bonuses or special materials)
If the weapon is a bludgeoning weapon, and the armor is made of chains, or likewise armors that would logically not offer resistance against blunt attacks, it uses the rules of piercing weapons when critting.
A magic weapon will ignore some DR, and still deal it’s normal damage. It follows the rules for magic armor for how much DR it ignores, which is half their enchantment level, rounded up. If the armor has a higher hardness than your weapon (both from special materials and enchantment bonuses) then your magic weapon won’t ignore any DR at all.