I am about to commit an act of heresy. I’m going to say that it actually does make sense for armor to be a to-hit modifier as it is in D&D. Say what? Dariel are you falling for that b-s- again?!
Well, consider what it takes to go through armor as opposed to going around it. I’m not saying the idea of armor as damage reduction is bad, but rather that the choice need not be exclusive. Most of the time, muscle-powered weapons will not cause significant injury unless they strike an unarmored part of the body. It takes a really heavy weapon, the strength to wield it properly, or better yet an energy source stronger than muscle –e.g. gunpowder – to truly pierce armor.
How can we model this? We have to regard armor now in terms of two properties, coverage and material strength.
Coverage refers to the amount of body protected by the armor, with a 15th century style full plate panoply including helm, arm and leg defenses as the ideal, down to single-piece partial armors like having only a Roman gladiator’s manica. This means each piece of armor will matter now. Helmets will add a bonus, as will shields, greaves, vambraces, pauldrons, gauntlets, etc. etc.
Let’s then consider Material Strength. This is the difficulty of piercing or cutting through the material the armor is made of, and takes account of both the material – leather, bronze, iron, etc. – and the construction technique used to build the armor.
I’m thinking, for simplicity, to give AC as a compound of Coverage and Material Strength. The base AC is from Coverage, then we’ll tack on bonuses if the material is good (assuming the base material is pretty soft, something like quilted linen or tanned leather).
You’ll have pretty decent chances to hit a barbarian in a leather vest and nothing else, but good luck fighting that antipaladin in full panoply of adamantium plate!
Changed Damage Dice
Another idea is to keep AC based entirely on Coverage, but change weapon damage dice based on the Material Strength of the armor. The premise is that if a hit did go through the armor, the armor would reduce the damage; or if it did bypass the armor’s coverage, the hit still went somewhere less vital.
So you could for example posit that damage for any weapon is 1d12 vs no armor at all, 1d10 for light armor, 1d8 for medium armor, 1d6 for heavy armor, and 1d4 for very heavy/very hard armor.
For critical hits, assume that the hit was made to an unprotected spot, so use the highest damage die.
Armor and Dexterity
What about the DEX bonus? I’d say it applies only if the character is wearing armor of a certain weight or less. Perhaps base it on character Strength.
If your AC from armor alone exceeds a certain proportion of your STR, no more DEX bonus.
This means that the light-armed swashbuckler is as valid a concept in combat as a tanklike knight, but they will have to fight quite differently.
With each piece of armor now important, you can play with tactics aimed at reducing your opponent’s armor coverage. Break or otherwise take away his shield. Get him to take off his helm.
You can also choose between defensive strategies. Will you risk going in with less armor, relying on your high Dexterity bonuses for protection but chancing more damage per hit, or will you trade a higher chance of being hit for a lower damage die?