Optional Rule Systems / Armor as Damage Reduction
Source Ultimate Combat pg. 192
In this alternative system, when a creature threatens a critical hit, it does not make a critical hit confirmation roll. Instead, the target of that critical hit makes a critical defense check instead. A critical defense check is 1d20 + a bonus equal to the creature’s DR + the creature’s Dexterity modifier (up to the maximum Dexterity bonus allowed by any armor worn) + the sum of any deflection and shield bonus to Defense.
Critical defense check bonus = creature’s DR + Dexterity modifier + shield bonus to Defense + deflection bonus
The DC of the check is based on the die roll for the critical threat. It is further modified by the base attack bonus of the attacking creature, how many critical feats the attacking creature has (if any; 10 maximum), and a bonus relationship between the size of the attacking creature and the target of the critical hit, if the attacking creature is larger than the creature it attacked.
Critical defense DC = critical hit roll + 1/2 attacker’s base attack bonus + 1 for each critical feat + 1 for each size category larger attacker is than target
For instance, if a Medium creature is hit with a crossbow fired by a Medium 6th-level fighter with two critical feats, and the critical threat attack roll is a 19, the target of the potential critical hit makes a critical defense check with a DC of 24 to reduce the critical hit to a normal hit. If the target is wearing +1 leather armor, has a Dexterity of 18, and is using a +1 buckler, that target would have a +9 bonus on the critical defense check to reduce the critical hit to a normal hit. On a roll of 15 or higher, the critical hit is reduced to a normal hit, and the target takes normal damage for the hit (which is reduced by its DR). If the creature firing the crossbow had rolled a 20, the target of the critical hit would need to roll a 16 or higher on its critical defense check to reduce the impact of the critical hit, making it a normal hit.
On a failed critical defense check, the target of the critical hit takes the damage for the critical hit. That damage is still reduced by the target of the critical hit’s DR. Fortification Special Armor Quality: The fortification special armor quality acts in concert with the check, coming into play if the armor check fails.