Can a target have two Armor Class (AC) values?

I had an idea for a kobold-operated tank.

The armor would be thick enough to be immune to damage from all mundane projectiles (arrows, bolts, flintlocks, javelins, etc.), and resistant to magic and melee damage. I have heard AC described as "how hard it is to hit you", and due to the size and speed (big and slow) it should have a low AC: it would be really easy to hit but really hard to do damage to.

I however wanted to add hard to hit weak points, like a pair of slits in the armor for the driver and ballista operator to see though, or the tank treads.

My question is this:

Can a target have TWO AC values? One low, easy to hit AC for the main body and a harder to hit but very vulnerable AC for specific weak points?

• What are you expecting a hit to an arrow slit to do? Hurt the guy inside? Because that sounds more like "rules for cover" and less "having two AC values". – Erik Oct 15 '18 at 17:27
• By "hard to hit weak points", isn't that the same as having a high AC? You wouldn't be trying to hit the non-weak points in the same way you don't try to hit your opponent's hair even if it is easy to hit, you try to hit their body, arm, etc. – firedraco Oct 15 '18 at 17:33
• What is the purpose of the "easy to hit AC for the main body"? Why would anybody target the main body, assuming it is "immune to damage from all mundane projectiles" and "resistant to magic and melee damage" (and has a lot of hp, presumably)? – enkryptor Oct 15 '18 at 17:49
• You can target objects being held, so why can’t you target separate parts of the tank. Barrel and treads aren’t permanently attached, if those are damaged the whole tank isn’t destroyed. See my post / comments for further explanation. – XAQT78 Oct 15 '18 at 19:50
• resistant to magic and melee damage - you are in the wrong edition of the game with this phrasing. Suggest you look up types of damage in the PHB and revise the question to incorporate how it works in 5th edition. – KorvinStarmast Oct 15 '18 at 20:49

There is a precedent.

Some creatures in the Monster Manual have body parts that, under certain circumstances, have a different AC from the creature itself. The Roper's Grasping Tendrils, for example.

But does that apply here?

Compare your theoretical kobold tank to a knight in plate armor. Is the knight harder to hit -- in the sense of 'make contact with' -- than a guy running around in his shorts? No, not at all. The armored juggernaut is the broad side of a barn, relatively speaking. The high AC he has comes from the difficulty of landing a blow that does any damage, because his heavy armor tends to absorb or deflect attacks harmlessly.

Similarly, the sides of a tank would be effectively invulnerable, and so an AC of, say, 22 could easily represent the difficulty of getting a hit in that actually goes through an eye slot, or wedges into a joint, or some such thing, rather than merely bouncing off.

Do whichever is more fun.

All that said, there's nothing wrong with deciding that different parts of the tank have different AC and HP values. It could be a fun fight where the tank itself has a ton of HP, so you can just smash your way through, but it has a series of 'challenges' that make the fight easier if the players are clever enough to use them -- like if you deal enough damage through the eye slot, it disables a weapon; if you deal enough damage to the wheels/treads, it stops moving, and so on. I think it's great to introduce a tactical element to the fight like that -- especially if you can achieve the same effects through clever use of skills, like a high enough strength check to wedge a crowbar into the turret so it can't turn anymore, or an Investigate roll that reveals a leaky fuel tank full of burnable fluid.

What I'm kind of thinking of here is Battletech, where each mech has a number of body locations you can aim for that each have different amounts of HP and different effects if you destroy them. The fastest way to win is sometimes to come right at your enemy and keep pounding away at the heavy frontal plating until it breaks, but other times you want to take the more difficult shots to aim for an arm or leg, to destroy a dangerous weapon or cripple its movement rather than just trade hits. (It works better for mechanical devices than monsters, who presumably feel pain and bleed and so on. Though now I'm thinking about a giant zombie fight with siege weapons and effects for causing arm damage or cutting a leg....)

• The Damage Threshold answer from enkryptor is more appropriate than an AC of 22. – Cœur Oct 16 '18 at 8:19
• @Cœur I don't think DT is really appropriate for character-scale objects. The example given for a DT-having object is a castle wall. Even a steel door should just use AC and hit points for a resilient object; a door isn't comparable to a fortification, unless you're talking about a massive portcullis or something. I feel the DT rules are more meant to be a tool to prevent players from thinking they can just cut their way through a stone wall if given enough time. – Darth Pseudonym Oct 16 '18 at 14:00

One AC value should be enough

AC isn't about how hard it is to hit you. It is about how hard it is to damage you.

See the PHB, page 14, "Armor Class":

Your Armor Class (AC) represents how well your character avoids being wounded in battle.

Your tank is supposed to be "really hard to do damage to", that means it has quite big AC. DMG page 246 suggests AC 19 for iron and steel. When an attack overcomes this AC, it hits a weak point.

Your tank is also supposed to be "immune to damage from all mundane projectiles". You can use the Damage Threshold to reflect this. See DMG page 247:

Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal.

A brief historical reference

The very idea of one target having several AC values is not groundbreaking. Prior versions of D&D had a few AC values for different situations, there were at least three of them — "normal AC", "flat-footed AC" and "touch AC". Your tank is slow but heavily armored, so it would have high "normal AC" (so it is hard to hit a weak point), but low "touch AC" (it is quite easy to hit its armor) in 3rd edition. 5th edition changes this for the sake of simplicity, now we have only one AC per creature1. As a DM, you can change this back, if that seems reasonable for you; I'm just saying that wouldn't comply with the 5e design philosophy.

1 Usually. The rules also suggest dividing Gargantuan objects and some creatures to smaller parts.

• D&D really doesn't do itself any favors with things like Damage Reduction. AC is how hard it is to damage something, but DR is also how hard it is to damage something. With so many other things representing difficultly to damage, people want to read more into AC than is there. 5th edition has smoothed some of this out, but some weirdness remains. – Michael W. Oct 15 '18 at 18:10
• @MichaelW. how DR is related to my answer? – enkryptor Oct 15 '18 at 18:12
• I was expanding on why "AC is how hard it is to damage you" trips people up even though it's a very simple sentence... Apologies if that wasn't clear. – Michael W. Oct 15 '18 at 18:27
• Homebrews are like the the 3.5 Murlynd’s Spoon: warm cardboard. Find something similar and change it. Making an Apparatus of Kwalish a Goblin Tank isn’t breaking anything. Treat the barrel as held weapons, the tread/wheels as equivalent to legs. You just have to categorize what the game mechanics intent was. Anything that targets a creature ability to move, can effect the tank. Based on its size, would be granted advantage etc. – XAQT78 Oct 15 '18 at 19:48
• A good example of one AC should be enough in this case is the Apparatus of Kwalish (DMG page 151) which has AC 20 and 200 hitpoints - and it's a thing that a group of cobolds could use as a tank. – Sumyrda - remember Monica Oct 15 '18 at 21:29

RAW? No, one target can't have multiple ACs.

A target can have multiple ways to calculate AC, but it has to choose one to actually use. There is no such thing as a "called shot" or similar in 5e, so AC applies to every part of the creature. It's part of the abstraction of combat.

There is nothing stopping you from doing this anyway

As DM you have free reign to make cool stuff. You want to make certain parts of the tank harder to hit? Go for it.

If it helps, you can imagine / design the harder-to-hit portions as distinct creatures that happen to be attached to the larger tank, so the PCs can choose to target them or the tank. There is also nothing stopping you from saying, "When this thing gets hit, this other thing happens", where the 'other thing' could be the tank's speed dropping to 0, or losing resistance, or whatever you want.

One method might be to treat the tank as an object, while treating the Kobold occupants as separate targets that have Three-quarters Cover from the tank (for +5 AC). For even higher AC, you could use say, the Kobold Chieftain stats (or have the tank crew wear better armor).

You can supplement the tank with the Apparatus of Kwalish as well as use the rules of Cover and Mounted Combat.

• Cover (PHB pg. 196)
• Mounted Combat (PHB pg. 198)
• Animated Object Spell (PHB pg. 213) (objects size & AC)
• Apparatus of Kwalish (DMG pg. 151)
• Injuries (DMG pg. 272) (varied limb damage)

Sounds like a fun campaign.

• Hi there! If you could elaborate a little more on the actual implementation of your idea, this answer could stand on its own and wouln't be half bad. It would also be even better if you've actually done this yourself and could speak to the experience! – Jason_c_o Oct 15 '18 at 20:32