Fixing Weapons vs. Armor Class

At the risk of turning this into a blog for commenting on Delta’s blog, here are some thoughts on a post of his back in March about the big error in the infamous Greyhawk and AD&D Weapons vs AC charts.

Basically, Dan observed that there’s a fundamental error in the way the chart was derived from Chainmail to convert it to the d20 “alternative” combat system in original Dungeons and Dragons (specifically the Greyhawk supplement), and that chart was just reproduced and elaborated on in AD&D. The error was in converting Chainmail’s chart showing with this weapon vs. this type of armor roll this number on 2d6 to kill the target into D&D’s roll this on d20 to hit the target, they forgot to adjust for the armor class! Basically, the difficulty of hitting the armor is baked into the Chainmail table, but it’s a completely separate consideration in OD&D, so that e.g. in Chainmail a mace has pretty much the same chance of killing regardless of armor (roll 8 or better), that’s presented in Greyhawk as a mace has no bonus vs. any particular armor so the mace gets worse and worse chance of hitting as armor gets better! Oopsie.

So what would a “correct” version of the Weapon vs. AC chart for Greyhawk look like? That is, one that preserves the logic worked into the Chainmail chart as to which weapons are better against which armors, which seem to have at least rough approximation of what weapons historically were preferred against which prevailing types of armor.

The Chainmail Man-to-Man combat chart looks like this:

	Armor Class
	No Armor	Leather	Shield	Leather+ Shield	Chain	Chain+ Shield	Plate	Plate+ Shield
Weapon	9	8	7	6	5	4	3	2
Dagger	6	7	8	8	9	10	12	12
Hand Axe	7	7	8	9	10	10	11	12
Mace	8	8	8	9	8	8	7	8
Sword	7	8	8	9	8	9	10	11
Battle Axe	8	8	8	8	7	7	9	10
Morn. Star	6	6	7	7	6	7	8	8
Flail	7	7	7	7	6	7	6	7
Spear	8	8	9	9	10	10	11	12
Pole arms	6	6	6	7	7	8	9	10
Halbard	8	8	8	7	6	6	7	8
2 Hnd. Swd	6	6	6	6	5	5	6	7
Mtd. Lance	5	5	5	5	6	7	8	9
Pike	8	8	8	8	8	8	9	10

The first thing we have to deal with is all the target numbers in this chart represent kills; in Chainmail there were no hit-points or variable weapon damages. So this chart represents both the deadliness of the weapon and its ability to penetrate various types of armor.

What I’m going to do is assume that the relative deadliness of the weapon is represented by the target number vs. unarmored men, while the penetration ability of the weapon vs the various armors is thus the difference between its “normal” ability to kill an unarmored man and its lessened ability to kill armored men. This normalization gives the following chart of how much worse a weapon is against the various ACs relative to its ability to kill an unarmored man; we’ll presume that in D&D that ability to kill an unarmored man is represented by the weapon damage, from d4 to d12 or whatever.

	Armor Class
	No Armor	Leather	Shield	Leather+ Shield	Chain	Chain+ Shield	Plate	Plate+ Shield
Weapon	9	8	7	6	5	4	3	2
Dagger	0	-1	-2	-2	-3	-4	-6	-6
Hand Axe	0	0	-1	-2	-3	-3	-4	-5
Mace	0	0	0	-1	0	0	1	0
Sword	0	-1	-1	-2	-1	-2	-3	-4
Battle Axe	0	0	0	0	1	1	-1	-2
Morn. Star	0	0	-1	-1	0	-1	-2	-2
Flail	0	0	0	0	1	0	1	0
Spear	0	0	-1	-1	-2	-2	-3	-4
Pole arms	0	0	0	-1	-1	-2	-3	-4
Halbard	0	0	0	1	2	2	1	0
2 Hnd. Swd	0	0	0	0	1	1	0	-1
Mtd. Lance	0	0	0	0	-1	-2	-3	-4
Pike	0	0	0	0	0	0	-1	-2

So here we see the relative values of armor against a given weapon. Against a mace, no armor really helps, though leather + shield is a tiny bit better than unarmored and plate is a tiny bit worse. Swords, though, quickly become ineffective against heavier armors, which take a two-handed sword to punch through. Mounted lances and spears are almost completely ineffective against plate + shield combination. This all seems plausibly historically accurate.

Finally, though, we have to convert this to d20, taking into account the way armor class is worked into the target number to hit on a d20 (the crucial step the author(s) of that section of Greyhawk forgot). This yields the following chart of modifiers that preserve the penetrating power of weapons from the Chainmail rules:

	Armor Class
	No Armor	Leather	Shield	Leather+ Shield	Chain	Chain+ Shield	Plate	Plate+ Shield
Weapon	9	8	7	6	5	4	3	2
Dagger	0	0	0	1	1	1	0	1
Hand Axe	0	1	1	1	1	2	2	2
Mace	0	1	2	2	4	5	7	7
Sword	0	0	1	1	3	3	3	3
Battle Axe	0	1	2	3	5	6	5	5
Morn. Star	0	1	1	2	4	4	4	5
Flail	0	1	2	3	5	5	7	7
Spear	0	1	1	2	2	3	3	3
Pole arms	0	1	2	2	3	3	3	3
Halbard	0	1	2	4	6	7	7	7
2 Hnd. Swd	0	1	2	3	5	6	6	6
Mtd. Lance	0	1	2	3	3	3	3	3
Pike	0	1	2	3	4	5	5	5

And here we see, as intended, against an unarmored foe a 1st level Fighting Man armed with a mace would need to roll a 10 to hit, and against somebody with plate + shield would need… a 10 to hit. (To Hit of 17 from the Men Attacking Matrix in original D&D, with a bonus on the roll of +7) Armed with a dagger he’d need the same 10 to hit an unarmored man, but a 16 vs plate armor + shield.

Is it worth it? Frankly, I have my doubts. If Gygax himself never bothered with it, it’s hard to see the added complication of the table lookup every time you switch weapons or foes (assuming they’re not all equipped identically) adds that much. On the other hand, it is kind of logical that you ought to prefer the weapons that were historically favored against particularly heavy armors. One thing that is clear to me, though, is that if you want to have that kind of mechanic in your game you’re better off starting with the Chainmail assumptions and not their mistranslation.