Ultimate Combat had the audacity of (re)opening that old tin can of worms “armors should reduce damage”, last seen on D&D (if I remember it right) at Unearthed Arcana… which is great! Variant systems are the kind of mechanic that I never tire of reading.
(You knew there’d be one, no?)
The problem with any optional system that turns Armor Bonus to Damage Reduction is that they mustn’t forget that no amount of DR can compensate for being hit and damaged at every single strike.
It also seems that many forget that characters that don’t use heavy armor become almost unplayable in these variant systems because the DR granted by light armor becomes practically irrelevant – and, to make things worse, light-armored characters are further penalized by the fact that any reduction on Armor Classes numbers make a huge difference in D&D/Pathfinder (in part because at mid and high levels, all strikes are usually hitting due to the great disparity between attack and defense).
So, how to address that? (That is: How to address that if you still want to use the rules as written on Ultimate Combat?)
Ok, some quick ideas that I had after reading them… I hope they might help you in this endeavor (these are just some random thoughts; I don’t pretend to post solutions here).
As you can note I’ve changed “Armor Class” to “Defense” here in order to avoid confusion (and because with armors acting as DR it doesn’t make much sense to keep the old terminology).
1) Double the Shield Bonus
This one is (I believe) from A Game of Thrones d20. In fact, I believe it can be used with other ideas below. It not only help to compensate for the reduced Defense, but also reinforce the importance of shields (which I like).
2) Distribute your BAB between Attack/Defense
At the beginning of a character’s turn, he must distribute his total Base Attack Bonus between Attack Rolls and your Defense stat. It does require a little math, but it helps to compensate the reduction to Defense in general. It also adds a nice tactical side to each turn.
A good rule of thumb to implement this is to establish a maximum to the amount of BAB “points” that can be distributed to Defense. I suggest that this limit must be your total BAB minus your total Armor Check penalty. For example: if you’re using a chain mail (armor check -5) and your BAB is +7, you can distribute a maximum of 2 points to your Defense (leaving your BAB reduced by 5 and your Defense increased by 2).
3) Use your CMD as Defense
This one is radical but simple. Use now your CMD as Defense, with the following extra rules:
Your CMD suffer a -5 penalty for each attack after the first in the same round;
You can add your shield bonus to your CMD.
An interesting side effect is that big monsters are now a lot scarier. You can add some tactical depth to this by ruling that any character can sacrifice a move action to ignore one “-5 penalty” do his Defense.
4) Add a Base Bonus to Defense, but use also Armor Check Penalty.
This one is radical and I’m not sure if it’ll work. It almost turns Defense into CMD. Don’t forget that in this variant your total Defense is 10 + Dex + BAB – Armor Check Penalty. Being proficient doesn’t change that because your armor is still cumbersome.
Another option is to establish a Defense Progression by class (a famous solution since Wheel of Time RPG). I suggest the following progressions:
12HD* and 10HD Classes: use the Medium BAB progression as your Defense;
8HD Classes: use the Poor BAB progression as your Defense;
6HD Classes: these classes gain a +1 to Defense at 5th level and every 5 levels thereafter (maximum +4 at 20th level).
*You can give to the Barbarian a Defense progression equal to its class level if he’s wearing light armor or no armor. Otherwise, just use the above progression.
If using this system I also suggest that the Monk lose its AC Bonus.
A 1st level Ranger (d10) would get a Defense bonus of +0.
A 5th level Fighter (d10) would get a Defense bonus of +3.
A 10th level Cleric (d8) would get a Defense bonus of +5.
5) Parry and Dodge Rules
When attacked and not flat-footed (or with the Combat Reflexes feat) you can parry. Parry works like using an Attack of Opportunity – in fact each parry consumes one of your AoO for that round.
A Parry works like an Attack Roll but you add your shield bonus to it.
Using this system I recommend that each character gains one extra AoO for each +6 at your BAB. I also recommend that Combat Reflexes gives you extra AoO based on either your Strength or Dexterity modifier (whichever is best).
You can also simulate Dodge with this system, but it requires two AoO and at least one space around the dodging character must be empty or he must fall prone after the attack.
To dodge rolls 1d20 + your total Reflex bonus +/- Size mod + any dodge, luck, protection or deflection bonus. You must score equal or higher than the Attack Roll of you enemy.