One of the more infamous bits of D&D's history is the Weapon vs AC chart. As presented it is cumbersome to use and doesn't mesh well for monsters. Plus with the addition of new armor types in the transition from D&D to AD&D it can be argued it doesn't even represent what it was originally designed for. The fact that different weapons have varying effect against different armor.
Fortunately other RPGs have developed more elegant ways to simulate this effect. Most of them have one thing in common. They distinguish between three types of damage, a blunt mass, a edged blade, and with a point.
Using the ascending AC system of Swords & Wizardry
Chainmail is normally +5 to Armor Class
Looking at how the different damage type work the armor class probably look like this.
+4 vs Blunt - the chain mail spread out the impact of a blunt blow but it is flexible and doesn't offer much resistance.
+7 vs Edged weapons - this is where chain mail excels, blunting the impact of a edge across the wearer's flesh.
+3 vs Point attacks - chainmail doesn't do so well in this regard. Enough of an impact the links can be shoved apart, allowing the point to penetrate.
The above may be logical it is not practical for a D&D style game to have varying Armor Class. We can flip this around and turn it into modifier for a weapon attack.
The AC remains at +5. But for the attack his modifiers not look like this
+1 if attacking with a blunt weapon
-2 if attacking with a edged weapon
+2 if attacking with a pointed weapon.
Note that you will see in the armor chart that most armor offer superior protection against edged weapons. However remember using blunt and piercing weapons comes at a price of reduced damage. If you get past the armor with a hit your opponent is going to be hurt.
Here is the full armor writeup. The modifier are bonuses or minuses to the attacker's to hit roller.
Quilt +1 AC, -2 blunt
Soft Leather +1 AC
Hard Leather +2 AC, -1 Blunt, -1 Edge
The rigidity of boiled leather offer good protection against blunt weapons.
Ring +3 AC, +1 Blunt, -1 Edge, -2 Point
The metal rings on leather greatly improved protection against edged weapon. But it's flexibility makes it little better than soft leather, and no different for piercing weapons.
Scale +4 AC, +1 Blunt, -1 Edge, +1 Point
The scales increase armor protection overall especially against edged weapons, but it is flexible so while better than ring doesn't get the full benefit, and piercing weapons can slip in between scales with only the leather to stop it.
Mail +5 AC, +1 Blunt, -2 Edge, +2 Point
see above for comments
Banded +5 AC, +1 Blunt, -2 Edge
This represents various coats of plate, roman Lorica Segmentata, etc. Better than chain versus piercing weapons but more expensive.
Plate +6 AC, +1 Blunt, -3 Edge
Plate is the king of armor with superior protection to just about anything. Blunt weapons do the best as the broad impact allow the rigid metal to be bent more easily degrading the armor's ability to protect the wearer.
This is good for people centric campaign (like my own Majestic Wilderlands) but overly complex if you are dealing mostly with dungeons and monsters.
If using this system then for monsters treat their AC like soft leather with the Monster's AC equally effective against all weapons. If you think something is obvious (like a reptilian monster's scale) then by all means treat like one of the above armor.
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