Thinking About Armor Class

As things gear back up for my B/X game I was having more thoughts on Armor Class , where it came from, and how to use it. 

Right there in the title we're in to old wargame design, where the Class is a number or letter code to let you know a) what the thickness of warship armor was and b) what column to cross reference on to see if a hit occurred. D&D and other F20 games just absorbed that even as the linkage between the Armor's Class and the chance to be hit attenuated. 13th Age even makes a joke of it, saying that it should be Armor Defense to go along with Physical Defense and Mental Defense, but they are traditionalists.

Now, I know almost no one used the weapons vs. armor tables in AD&D. I know I didn't... but I am a fan, because I love the implied physics engine. (Something you'll see more of in my V&V natter later; there's a similar one in early Gamma World.) It was just too complicated because there were too many weapons and too many armors and Armor Class lost all meaning because the AC on your sheet wasn't the Armor Class on the chart, which was just your armor, or maybe armor and shield (which also got weird because AC 7 could be Studded Leather or Leather and Shield, which doesn't make much sense on how weapons interact with the armor). 

So lets strip this down to B/X levels and my own "get the d20 roll to match 1 in 6 chances" ethos. There are 5 "Armor Classes", that is to say, broad classes of armor.

1. None: This is everything from nudity to general clothing. Unless the clothes are constructed with the goal of blocking weapons, your Armor Class is "None". 
1. For animals, this is light fur; everything from cats to dogs to cows and horses are Armor Class "None" 
2. This gives a bonus on stealth and physical action. 
3. An average opponent hits you 4 times in 6. (8+ on d20)
2. Leather: This is any sort of animal products being used to protect the bulk of your body, with or without some metal studding, of which there are a million variations. 
1. For animals this is heavy furs and moderately thick hides, such as wolves and bears. 
2. This gives you a bonus on stealth and no penalty on physical action.
3. An an average opponent hits you 3 times in 6. (11+ on d20)
3. Chain: This intermediate stage is, for humans, interlocking chains covering at least your torso backed with some leather or hard metal in form of greaves, bracers, etc. 
1. For animals this is rhinoceros and elephants, or large lizards where the scales approximate chain, or insects with basic exoskeletons.
2. This gives you no bonus to stealth and a minor penalty on physical action if you don't have specialized training. 
3. An average opponent hits you 2 times in 6. (14+ on d20)
4. Plate: This is classic conception of "Knight" or "Royal Guard", where the bulk of the body is covered with solid metal backed by chain mail. 
1. For animals we're now talking giant insects with natural carapaces, younger dragons, and other thing that have that level of protection. 
2. It gives you a minor penalty to stealth and a major penalty to physical action which is reduced to minor by training. 
3. An average opponent hits you 1 time in 6 (17+ on d20)
5. Adamant: This final category is for magical plate armor. It is nigh impermeable. 
1. For animals this is elder dragonscale, or extraplanar creatures
2. It gives you a major penalty to stealth and and a minor penalty on physical action if you don't have specialized training.
3. An average opponent almost never hits you (20 on d20)

This seems self explanatory, and lends itself easily to naturalism: lots of animals will have Armor Class None or Leather, you can easily eyeball threats as they come up. Now... how much complexity do you want to add? 

Shields: Ugh, I love them conceptually, I hate the idea of of them messing with how weapons interact with the armor, and I'm meh on having them affect the number needed to hit you. So what to do with them? 

1. In my current rules hacks, Fighters gain bonuses in combat based on their raw d20 rolls, as per 13th Age. Rolls of 1-7 give them an informational or defensive bonus, rolls of 17+ give them an attack bonus, determined by what style they're fighting with. 
1. Weapon and Shield is already a style here, so there's already a mechanical reason for fighters to have a shield - fighters want to unlock those benefits
2. I can add a 1-7 benefit that is only accessible for shield carriers, to give another reason to carry them which feels diegetic. 
3. The problem is that these are Fighter only benefits, but Magic Users and Rogues don't use them. So why do Cleric carry shields? Cleric is a pain because it's such an amalgamation - it's scholarly magic and divine channeling and armor using combat - that it's hard to split apart. Since I'm already planning to make the current 'Cleric' a multi-class off the scholarly magic class and the divine channeling class, the divine channeling class needs some sort of shield bonus. 
2. There's a lovely rule in Feng Shui section edition for bulletproof vests where they don't reduce damage or change your chance to hit, but give you a chance to pop back up from an otherwise fatal wound. Likewise, the Magic Shield in 13th Age just give you more Hit Points. 
1. A similar to Feng Shui rule here, where the Shield lets you evade death, might be helpful. But maybe too helpful? Or too complex? The Shield let you eliminate the damage from one attack per fight, or two attacks if you let the shield break and renders it unusable for later fights? That would work for Fighters and "Prophets"
2. A similar to 13th Age rule might also work, where carrying a shield just gives you 1.5*level more HP, but how does that effect healing. Given my own healing rules are idiosyncratic that's less of a concern as it hard to do in dungeon healing. 

Now what about Dexterity Modifier? That also changes the "to be hit" threshold of your Armor Class. That's easier. Dex Mod adds to your Hit Points, just like Constitution. Hit Points are, to me, "Don't Get Hit" points, the reserve that turns what would have been a fight ending hit into a lesser one. The pool is an indicator of your tricks and skills that minimize attacks, but you get tired and run out of tricks eventually. Constitution modifiers show how much pain you can take that might put other people out of a fight. Dexterity shows how much faster you are than other people and gives you a deeper bag of tricks. Make that change and the rules for Armor get a lot easier because it's not BOTH dodging and blocking, It's just blocking. Dodging is back into HP.