Early on in my gaming career, I adopted Arms Law into my campaign. I thought the idea of more armor making you easier to hit, but you taking less damage, was a neat twist.
However, to me the biggest drawback was needing a separate chart for each weapon. While this was certainly cool from the standpoint of every weapon being custom designed for every armor type, I found it a bit cumbersome.
I soon took to designing my own system, inspired by the same basic principle. I have used it for many years in my campaigns, and am quite happy with it. Among other things, it encourages players to wear lighter armor, and works weal with adventures that require stealth. In addition, I find that there is a lot more control over weapon and monster design, making a monster virtually impervious to weapons, or having sneak attacks that deal dozens of points of damage on the unarmored and unaware.
I'll start by introduction the character sheet:
We have a fighter, Sebastiano, who has an impressive array of statistics. Generate stats your favorite way.
Then, indicate your d20 attribute bonus. The following column is the d100 attribute bonus. This is simply the d20 attribute bonus, x5. His Strength of 18 gives him a d20 bonus of 4, and a d100 bonus of 20.
He has 27 hit points, which is also his "Combat Rating", or CR. His hit points total at zero level was his Constitution or Stamina, divided by 2, giving him 9 points. He rolled a 7 at level 1, and a 5 at level 3. At each of those levels he was awarded his d20 bonus of 3.
Next, we have to pick armour. For this we need an armor weight table. Sebastian is going to wear a light plate (AC4) breastplate, bracers, and helmet. For movement and stealth purposes, he will wear leather (AC8) on his legs and upper arms. A light plate cuirass weighs 18 pounds. The helmet is 2 pounds. The bracers are 4 pounds each. His leather armor weighs a total of 5 pounds. His armor comes to a total of 33 pounds.
His Combat Penalty (CP) is calculated by adding up all the weight he is carrying, and subtracting his weight allowance. His armor and weapons come to a total of 43 pounds. He is allowed a standard 20 pound weight allowance, and his strength d20 bonus gives him another 20 pounds of allowance. His CP is 3.
Calculating hit probability is quit simple. The Attacker's CR is added to the Defender's CP. So, if Sebastian was fighting himself, he would have add his CR (27) to his doppleganger's CP (3) for a "to hit" score of 30. He has not yet used his Dexterity Bonus. For simplicity, we will say his Doppleganger gets no dexterity bonus. Sebastian has 15 points to play with, which is his d100 Dexterity bonus. He has to keep it in units of 5, so he will put 5 points into defense, reducing his CP to -2, and He will put the other 10 points into offense, bringing his total to hit to 40.
If he rolls a 40 or lower on d100 he hits. His doppelganger has a CR of 27, and Sebastian has a CP of -2. His doppelganger needs a 25 or lower to hit him.
Sebastian rolls a 33, hitting the doppelganger. Now we need to know exactly where he was hit, so we can calculate damage. Rolling a d100 and consulting the Armor Hit Location table on the left of the Character sheet, we get a 62. This number is between 56 and 95, so a hit to the torso is indicated. The doppelgagner is wearing light plate on his torso.
Next, we need to calculate damage, and for that we will need the damage chart. There is only one chart required for all weapons.
All damage for all weapons is rolled using a d10. Strength and weapon modifiers are added to the roll.
Sebastian rolls a 4. His sword does not have any damage modifiers, but he gets a +6 to damage due to his strength d20 bonus (+4) and his use of a two handed weapon multiplies his bonus by 1.5
6 is added to the 4, for a total of 10. This is cross indexed with the light plate AC4 column, for a total of 6 points damage. Had the attack hit an area that was covered by AC8 leather, the damage would have been 14 points.
For each 10 points of damage dealt in a single blow, an opponent is stunned for 1 round. The opponent can not counter attack, and the attacker gets a +10 bonus against the stunned opponent. If the attacker went first in the round and the opponent loses his counter attack in the round due to the stun, the following round he loses initiative and is subject to the +10 bonus against him. If the stunning attack occurs at the end of a round, the stunned opponent is stunned for the entire following round.
That's pretty much it. I believe I've covered all the bases. To calculate damage in excess of 27 points, simply add the damage and the AC together, then multiply by the AC/10 and round .5 up to the next number. Lets say 32 points of damage were done on AC4. 32 and 4 are 36. 36 x .4 = 14.4, which is rounded down to 14.
Sebestiano vs. a housecat
One way to evaluate a combat system is to set an opponent against an average house case. Consulting the weapons chart, my housecat has a CR of 40, with a damage modifier of -19. The cat must succeed in successful claw, claw, and bite attacks to roll once for damage.
Lets say poor Sebastian is sleeping one night, and is attacked by a house cat. Seeing as the cat surprised him (+20) and he was prone (+20), the cat might get an additional +40 to hit. Rolling 3 times and getting results under 80, the cat can roll for damage.
The cat can roll anywhere from 1 to 10, so his modified roll is anywhere from -18 to -9. Looking at the armor chart on against AC10, the cat actually has to roll a 10 to do 1 point of damage. Anything lower than 10, and there is no damage. It is impossible for the cat to harm someone wearing any type of armor on the area of attack.
More examples soon. Feel free to play around with the concept, altering numbers as you see fit. Generally, for each +5 a weapon gets to hit, it gets -1 to damage, and vice versa. Some weapons are worse, but making a weapon too much better creates a super weapon.
Finally, here's a blank character sheet for your use: