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?
Best Answer
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....)