I have an entire system with a weapons to armor adjustment, but I put armor in three categorys light, medium and heavy.
Light is armor that adds 1-3, medium 4-6 and heavy 7-9. I added a couple of medium armor types; lamilar and brigadine which are both at AC 5 and bumped chain, breastplate, and the heavy armors up by one. Using categories such as light, medium and heavy also allows adding natural armor class creatures into it as well.
Light armors as a whole are just cloth and leathers with perhaps a little metal trimming.
Medium armors present a little bigger problem and my system becomes less perfect and more vague. Medium armors comprise of leathers, chain, scale and plates. With such a wide variety of possibilities about the only thing you can be pretty certain of is that one's entire body isn't likely to be covered and many armors have more than one primary component.
Heavy armors may or often combine a few different elements, but at least it's mostly plate and usually near full body coverage.
Just add three numbers by your armor on your character sheet such as +1/0/-1 for instance. This signifies against slash, pierce and blunt. It can be that easy and simple.
My weapons chart took a much longer period of time though. It includes unarmed attacks, differences between using wooden and metal blundgeoning weapons, differences between chopping and slashing weapons, differences between hafted piercing weapons such as pics and thrusting piercing weapons and difference in weapon sizes. I also include shield bashing in the mix and modifiers for bows, crossbows and all other ranged weapons.
Everything is very systematic and kept the idea of the +2 ro -2 circumstance modifiers in the 3E system. I didn't want to effect things too drasticly and therefore is not always completely realistic, but there are differences and play balance isn't too warped. Examples would be that a person attacking a someone in full plate with a fist or small wooden club wouldn't get very far not to mention a whip (unless they disarmed, or tripped them). I didn't want to make these attack options worthless just hindered in comparison to most other options.
Something else to consider is that some weapons are not the easiest to wield and lighlty armored, or unarmored targets may fair well by getting out of the way. Getting a +3 to hit a naked person with weapon X doesn't seem fair. Penetration values will not effect the person with no armor.
A started with blundgeoning, slashing and piercing. I also broke things down into small, medium and large weapons. Blunt weapons were very good vs heavy (+2), good vs medium (+1)and average vs light (or unarmored) (+0). Or +0/+1/+2 on the character sheet by the weapon.
A light blunt weapon would be -1 from the above bonuses vs medium and heavy armor. There fore a +0/+0/+1 beside the weapon on the character sheet.
A large (two handed) blunt weapon would be a plus one to the standard. -1/+2/+3
All of this assume using a blunt weapon with a metal head. If using a wooden weapon subtract one and if using flesh (a fist) subtract two.
My objective was for certain weapons to have the ability to cut down medium and heavy armor types, but I did raise two of the medium and all of the heavy armor types. There are also weapons that actualy get a negative to heavy armor's so in this medium and heavy armors do not actualy get the shaft.
I also give a negative to large two handed weapons vs. light (or no) armor types to reflect the unwieldiness of large weapons vs quicker moving targets. Mostly I didn't want to take away the only armor bonuses light armored characters had and there fore help to retain game balance. Other wise all large weapons will reduce most light armors to nothing or nearly nothing.
The last two paragraphs are there to explain my person skew/spin and objective on the whole issue.
It may seem complicated reading this but all you have to do is make a tic tac toe thing with 9 boxes. On the left side of it from top to bottom is "small, medium and large" for the weapon size and "light, medium and heavy" across the top for armor categories. This will help keep your weapons consistant with one another.
I have slasing at being better towards lighter armor types with chopping weapons like axes being in between the blunt weapons and slashing. I put piercing weapons that are hafted and swung with an arcing motion in the same category as blunt weapons (vs heavy armors) and thrusting piercing weapons are good vs light and medium armors. Blah, blah, blah.
The charts take up about a page in excel or a little more and every weapon can be placed in a category. Special consideration can and probably should be made though for some weapons. A rapier is listed as a medium weapon, but it doesn't have a strong base like a dagger and the long thin blade makes it not as effective vs heavy armor when compared to a spear, or lance. Some weapons are listed as large only because they are normaly wielded with two hands, but shouldn't get the same consideration as a normal two handed weapon. A quarter staff for example is not normally wielded the same as a two handed maul. I consider a quarter staff to be a medium weapon for these purposes.
I'd post the whole charts, but I don't know how to box thingy's on these sites.
One nice thing about having modifications to just the armor types and not just the weapons is that characters are sometimes attacked by by creatures with natural weapons and I don't want to have to figure out what the modifications are for natural weapons. I just use the modifiers for the armor type vs the type of damage being done.
When a weapon is used then I use the weapon modifiers vs Light 1-3 AC, Medium 4-6 AC and Heavy 7-9 AC and so that includes natural armor as well.
I do not combine the two together because I found that it unbalanced things game mechanic wise too much for my taste even though it may have been more realistic.
Example: If I have Full Plate armor at +1/0/-1 (slash, blundgeoning, piercing) and a Warhammer at 0/+1/+2 (light, medium, heavy) and a Longsword at (+1/0/0) it all adds up pretty fast.
The Full Plate wearing warrior has an effective +1 AC vs the longsword, but a -3 vs the Warhammer. That's a spread of 4. How many people want to wield a Longsword now? By only using the armor modifiers vs natural weapons and only using weapon modiers when weapons are used the same situation would be:
The Longsword would have no modifier vs. Full Plate and the Warhammer would be at +2. A spread of only 2 now. A reason to wield a Warhammer, but not all characters will throw away their longswords. (Longsword would have a +1 vs lightly armored and non armored targets as well).
Sample weapons:
Longsword +1/0/0
Warhammer 0/+1/+2
Shortsword +1/+1/-1
Club 0/0/0
Greatsword 0/1/1
Flail, heavy -1/+2/+3
Sickle 1/0/-1
Unarmed, medium 0/-1/-2
Longspear 0/+2/+1
Just some idea's. If nothing else it might get some wheels spinning. If you don't like the idea of giving a -1 to large weapons vs light armor types that might not be a bad one to drop. I'm considering it myself, but it's a nice game balancing thing.
Here again. I wasn't shooting for total realism, but I wanted there to be differences between the weapon and armor ratio, but not too much.
Slashing I put as good vs light. Swords are finese weapons and so are thrusting weapons in general. Thrust/piercing weapons are good vs most medium armors because they slide in between plates, scales and espcially chain. Blunt and hafted piercing are good vs heavy and chopping seems to be in between blunt and slashing. Not as acurate as a sword, but packing more punch. I made small weapons better vs light armors as a whole since they are fast and easy to control and large weapons clumsy vs light armors, but better at penetrating heavy armors. I had a few ideas in this and put them together.
Oh yeah. For the purposes of this realize realize that a chain shirt is considered a medium armor since it falls in the 4-6 range and hide armor is considered lite in the 1-3 range. This means that mace, hammers and picks as well as a few other weapons will be more effective vs the chain shirt and the lousey hide armor at least gets to duck under the medium category and avoid this.
Book over.
