Originally, in this post, I explained a small modification to the known AC system used in D&D up to advanced second edition. My goal was to reintroduce something that was lost when the game migrated from wargaming to RPG, but then, as a final afterthought, I made some calculations and discovered that, well, my changes made little (although not insignificant) difference. Preceding a post with a disclaimer like “what you are going to read may not be as useful as it seems” is probably not the best hook, but I still believe there are a few interesting bits here and I may also have unwillingly solved an ancient argument about AC vs. damage reduction that sometimes still rises from its grave (spoiler: there is surprisingly little difference in the long run unless you make a completely different system from scratch.) Besides, I’m a believer in the idea of publishing negative results, even if they are not as eye-catching as positive ones.

Having said that, here’s the post:

If I remember correctly, the idea of Armor Class was something Gygax and co. took from an old naval wargame, where each type of ship had an “armor class.” It’s a very useful concept to represent how hard is to damage, wound, or disable a target, as in: you need to pierce through armor to reach its inner machinery or squishy bits. The keyword here is target, which is passive. It actually works quite well in wargaming, although that’s not where most people see AC being used. For example, if 60 archers shoot some orcs, increasing (decreasing, using the classic system) their AC by giving them better armor naturally would decrease the number of casualties (and, remember, casualty doesn’t mean dead, just that the unit cannot fight anymore.) That’s common sense, and it works really well within the tactical view of wargaming, where almost all units have a single “wound” or hit point.

It works well too when we are talking about hand-to-hand combat and especially if big numbers of troops are involved, like in the previous example. Everything else being equal, if two units are fighting, the one better armored should last longer, which naturally would increase its odds of winning. But here’s the issue: when you go from that sky-high, statistical view to the more individual level of one-to-one duels and when characters don’t have a single hit point but many, something odd happens from time to time, even if only in terms of how things are understood or explained as they happen in-game. Imagine this:

Two man-at-arms NPCs are fighting, armed with nothing but swords. To simplify things, let’s assume they have 1hp and no DEX bonuses. Therefore, whoever manages to prick the other one first, wins. But what happens if one of them is wearing a heavy plate armor?

The obvious answer is that the other guy is screwed, and that is correct as far as the combat result and the dice rolling are concerned, but… why? It’s not because the armored man is a better fighter, for his fighting capability (THAC0, attack base, whatever) is the same. What has changed is that the other fighter will need to be extremely lucky to hit him, so, in the long run, the well-protected one will win. Basically, while the armored fighter is playing a coin-tossing game to see if he wins, his opponent has to roll a d10. Statistically speaking, his victory is almost inevitable.

Now, I have no problem with this system, and as said before, it works well in wargaming, but there we are talking about undifferentiated masses of soldiers and turns/rounds that are supposed to represent something like 10-30 minutes each.

But when we are dealing with 1 vs. 1 combat and rounds lasting 1 minute or even just a few seconds, I doubt that’s how the fight would look like. In fact, in “real life,” and as far as I can guess how these things would look like, the armored man would fight better, and the most obvious result is that the fight would end way sooner, while standard D&D rules say it should last the same or even longer. In a real fight, the well-protected soldier is basically a tank, so he just has to rush the other guy and crush him with his 30 kg of armor. He’s basically a human rhinoceros. It’s not just that he is well-defended, as a target against arrows or blows that he passively endures, but that his whole fighting capability has increased because he now can focus on the offense.

When two unarmored opponents are fighting, the goal is to avoid the other as much as possible and to strike fast, for every mistake can be lethal. Sure, that happens in armored combat, but the technique and possible openings change. While you (unarmored) are still dancing around the guy inside the heavy armor, the other (even if he has no idea how to wield a weapon) can just, well, rush you. What are you going to do? Stab him? Once one fighter appears with a heavy armor, the whole dynamic changes, and not just because that person is better protected.

In other words, armor is also a weapon; and the etymology agrees with me, by the way. To simulate that, while retaining a certain sense of balance (not much, though, for I care not about it) and avoid overcomplicating house rules that for every thing they fix they create ten new problems, I have broken down the known AC into three components (although two of them are the same.) Also, since people making up their own rules is pretty much a staple of RPGs, I cannot claim I’m the first one to do this.

(Descending AC)

No Amor: 10 (10/0)

Leather or padded: 8 (9/1)

Studded leather or similar: 7 (8/1)

Scale mail, brigandine, and similar weirdness: 6 (8/2)

Coat of mail: 5 (7/2)

Splint mail, banded armor, etc: 4 (6/2)

Plate mail: 3 (6/3)

Field plate (whatever that is): 2 (5/3)

Full plate or bestest armor in the world: 1 (4/3)

Shield: -1 to whatever base AC (-1/0)

What does this nonsense mean? The first number is the usual AC we all know and love, but you are not going to use that now. Of the two numbers in the parenthesis, the first one is the new AC, and the second number is both your damage reduction and your bonus to your attack, assuming an unarmored opponent. What this means is that when in melee, the opponents compare the second value, and the difference is added as an attack bonus to the winner. So, if some naked fool tries to duel a well-armored knight, the knight would get a +3 to attack. This represents the fact that the guy with the best AC has a bigger range of offensive possibilities, a greater sense of security which allows him to attack in a more straightforward manner, and lunges and techniques that would have ended up with him impaled may now be very useful. In other words, his task is easier.

I’m sure this system sounds off to contemporary brains, people so used to the idea of armor = defense that what I’m saying here may sound like pure nonsense, but as mentioned before, armor as weapon is one of the basic ideas of wargaming, from which almost all RPGs trace their origins. If you pick a game like Chainmail or WRG (Wargame Research Group) you will see that fighting capability is based on Types of units, which is determined by tactical function and equipment (heavy cavalry, heavy infantry, skirmishers, lancers, light infantry, etc.,) not training or expertise (what we’d call “levels.”)

Now, a few important things must be said now. One, it’s obvious that now hitting someone is easier than before, but damage reduction means damage is lower. Second, that may make you think I have unbalanced the game, but that may be just an illusion. For example, under the traditional system, a simple archer (THAC0 19) attacking the best possible armor (AC 1) has 15% chance to hit. With damage 1d6, that’s an average of 3,5 per successful hit, or 0,525 for each attack.

Now, with this new system, the same THACO 19 against the same armor (now AC 4/3) has 30% chance to hit. But with damage 1/1/1/1/2/3 (because of damage reduction 3,) the average is 1,5 per successful hit… or 0,45 per attack. Lower, yes, but not very significant. So all of those people who say D&D is broken because it has no damage reduction, well, at least according to this example, the result is practically the same in the long run.

What about big monsters and high-damage weapons? Well, let’s see. According to the AD&D 2nd edition Monstrous Manual, a normal troll has THAC0 13, and +8 damage if it uses a weapon (which rarely happens,) a spear for example. Against traditional AC 1, that’s a 45% chance to hit, with an average of 11.5 points of damage, or 5.175 per round. Against AC 4/3 armor, the creature has a 60% chance to hit, with an 8.5 damage average, or 5,1 per round. So, basically, the same.

But I also added a “bonus to attack” effect from wearing armor. How does that affect gameplay?

Using the first example of 2 fighters (THAC0 19, 1d6 damage, AC 10,) that’s 60% chance to hit, or 2.1 damage per round. If they happen to have 8 hp, that means the fight should last 4 rounds or so on average — which is enough time for a long, cinematic fight, for those who claim that low-level D&D isn’t cinematic or epic enough.

Anyway, if one of the fighters had AC1, that well-armored guy would still need the same number of rounds to win, BUT the other fighter would now have a 15% chance to hit (each AC increase/decrease reduces your chance to hit by 5%,) with 3.5 damage per average, or 0.525 per round (or almost 20 rounds to win.) Using my system, against AC 4/3, the unprotected fighter would enjoy a somewhat better 30% chance to hit, but with a lowly average of 1.5 per successful hit, or 0.45 damage per round. Lower, but, again, not a great difference.

However, the well-armored fighter has a +3 bonus to attack (or a THAC0 of 16,) which means he has a 75% chance to hit (up from 60%,) with an average damage of 3.5, or 2.625 per round. That’s a bit higher than the previous 2.1 (close to a 25% increase in effectiveness,) true, so instead of 4 rounds he may only need 3, but the difference is lower than I had expected. Of course, you could increase these numbers by increasing the attack bonus: as an example, if the attack bonus were the difference between AC (10 – 4), that would elevate the fighter’s chance to hit from 60% to 90%, with a per round average of 3,15. That seems too overpowered for me, so unless you are playing a system where equipment is everything, I wouldn’t recommend it. Still, it would be an interesting experiment to see how a game where AC is actually a combat bonus works like.

So, what does it all mean? That the original D&D combat system is surprisingly robust while being simple, and that the complexity that other systems build on top of it may be superfluous. The system I have presented here does add some significant differences, true. For example, well-armored first-level characters will probably not die from a lucky hit even if, in the long run, their chances of survival remain more or less the same. The biggest change is the bonus to attack due to armor difference, which combined with the somewhat worse results unarmored fighters get here, may significantly increase the importance of armor or the value of hordes or first-level, full-plated hirelings, but even in that case, the difference is not game-breaking, so someone who prefers simple rules (while leaving complexity to the imagination) may ask that what’s the point then.

Having said all that, and for those who liked this system, here are some expanded rules for the most common situations:

Shields: I know that in that fabled “real life” shields are also used as a weapon, but so does a free hand, as any hair-grabbing fighting chick knows. So I decided that these things cancel each other, which means shields don’t give any combat bonus nor damage reduction, just the usual defense bonus (although I’d probably double it against some projectiles.)

Critical hits: That’s up to you, but a useful rule to use here is that critical hits make the maximum damage possible and ignore damage reduction.

Magic Armor: The first +1 is added to the usual AC, the second to damage reduction, the third to AC again, and so on. These bonuses never add to fighting capability because they are of an ethereal or nonmaterial nature, and your enemy may not even know your armor is magical anyway. Also, it would be overkill.

Optional rule for damage: I had a fairly complicated table for this, with different rates for different weapons (well, type of damage, like penetration, blunt, hack, lasers…) but the basic idea is that every 3 points or so that you roll above the enemy AC, you add 1 extra point of damage. Useful for those who may whine about not being able to “snipe” well-armored fighters now or who want to play Legolas.

Monsters and enemies’ AC: The same rules apply to enemies and creatures, except construct, non-intelligent undead, and similar imbeciles. A golem or a zombie doesn’t care what armor you (or itself) are wearing, so these creatures don’t benefit from any potential bonus attack due to their AC (you still do against them, though.) When trying to decide what new AC any enemy should have, take the original AC and subtract one for leathery-like very tough skin or hides, two for metallic-like skin, and three for exceptional, dragon-like scales. This number should be used as armor reduction and possible attack bonus if you want to apply this rule. Higher damage reduction values are not recommended unless we are talking about… titanium golems or something like that.

Magic weapons: It’s up to you, but you can assume magic weapons (or some, anyway) pierce through a level of armor per each magical bonus.

Magic damage: Most magic damage should ignore damage resistance, but some may not if they are very similar to physical damage. Use your common sense.

Magic defense: I guess some spells could give both traditional AC (like Shield) or AC and damage reduction (like Mage Armor) but that’s up to you to improvise. Spells that give an invisible or immaterial protection should probably not give any bonus attack, though, but those that seem to change the nature of the target (e.g. stone skin) maybe should.

Penetration: It’s possible that some unique weapons, like a crossbow or that gauss rifle you found once, could ignore one level or so of damage reduction, but I haven’t given much thought about it. One amusing, but totally optional, possibility is to give fine-pointed weapons like daggers or rapiers one level of penetration against level 3 armors as they were known to be useful against well-armored targets, to get through the plates and that sort of stuff.

To sum up: use whatever you like from what I have written and ignore, change, or replace whatever you think goes against common sense.