What is Armor Class used for?

I have never understood or found any answer to how to use your AC. Is it something the GM have to keep in mind all the time to reduce or add damage to specific players based on how high their AC is, is it something you can use to calculate and reduce the damage received to a player, or is it just a number displaying what sort of defensive items you have on you? If so, why do you have to use armour and shields?

What is the AC of a character with 16 dex combined with the spell Armor?

The Spell Armor (Unearthed Arcana p.51-52) states:

The caster creates ,a magical field of force which serves as if it were leather armor (AC 8). If the spell is cast upon a person already armored, it has no effect. However, if it is cast upon a creature with an armor class normally better than 9 (due to its size, speed, skin, etc.) it will benefit the normal armor class by one step, i.e. AC 8 becomes 7, AC 7 becomes 6, and so on. The magic armor spell does not slow or hinder movement, adds no weight or encumbrance, nor does it prevent spell casting(…)

Normally,a character with 16 dex + a leather armor would have AC 6, what bugs me is the ”etc.” in the examples, I believe it is fair to say that my magic-user would have AC 6 since a comparison can be made with the real armor that it imitates, and ”etc.” might be other examples that I’m not aware of, but I don’t think the AC from dexterity is included in those examples.

In short:

Is my AC 6 or 7?

What is the long term interaction between grapple and Armor of Agathys?

Armor of Agathys (PHB 215, emphasis mine):

A protective magical force surrounds you, manifesting as a spectral frost that covers you and your gear. You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

Grappling (PHB 195, emphasis mine):

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them.The target of your grapple must be no more than one size larger than you and must be within your reach. Using at least one free hand, you try to seize the target by making a grapple check instead of an attack roll: a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you subject the target to the grappled condition. The condition specifies the things that end it, and you can release the target whenever you like (no action required).

I believe that grapple counts as a melee attack that activates Armor of Agathys. (Evidence in Bold)

If so, what happens on consecutive rounds of grappling? Assuming the attempt to escape has failed, and Armor of Agathys can still cause damage to opponents who hit it with melee attacks, does Armor of Agathys inflict more damage to the grappler on consecutive rounds of being grappled? Or is it just the initial grapple attempt that activates the spell and causes damage?

Is there an optional or house rule for effectiveness of specific weapons versus specific armor types?

In many epic fantasy worlds (as well as in the real world) there are protective measures, designed to protect from specific threats. For example, chainmail provides good protection against slashing attacks, but is not so good versus blunt weapons.

I know D&D isn’t a reality simulator, but, for the sake of diversity, I want to use a houserule that generally makes a particular weapon type more effective against a particular armor type.

To avoid reinventing the wheel, is there a well-known houserule (or an official rules variant, maybe) for making particular kinds of weapons more effective against specific types of armor?

Artificier Repulsive Armor Infusion

Within the Ebberon (ERLW) Text, the Artificier gains fanciful infusions.

At 2nd level, you gain the ability to imbue mundane items with certain magical infusions. The magic items you create with this feature are effectively prototypes of Permanent items.

One of these infusions is called Resistant Armor.

While wearing this armor, a creature has resistance to One of the following damage types, which you choose When you infuse the item: acid, cold, fire, force, lightning, necrotic, poison, psychic, radiant, or thunder.

The issue lays with the fact that you can only ‘learn’ this infusion once.

Unless an infusion’s description says otherwise, you Can’t learn an infusion more than once.

Does that, mean that each time I use it (after a long rest, only one Resistant Armor infusion in existence at a time) I can decide which resistance the armor takes on?

Or does it mean when I learn the infusion, that I have to choose a resistance, and can only use that one specific damage type whenever I use the Resistant Armor infusion?

I ask because with a different infusion that has multiple options, the wording is

REPLICATE MAGIC ITEM Using this infusion, you replicate a particular magic Item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables below.

I understand that specific beats general which leads me to believe that Resistant Armor allows the caster to choose (and change after a Long Rest) the resistance.

Can you wear armor over an Aegis Astral Skin?

Can you wear armor over an Aegis Astral Skin?

Astral Skin: When formed in this way, an astral suit resembles a psychoactive skin.

Psychoactive Skins: A deployed skin completely covers the wearer and all his equipment like a second skin, allowing the wearer to see, hear, and breathe normally. It rolls away from various parts of the body as needed, such as when the wearer needs to eat or access a backpack. Held items or items specifically excluded are not covered by a psychoactive skin.

Now does the statement “completely covers the wearer and all his equipment like a second skin” prevent armor being worn over it. The Astral Skin give no armor bonus unless Custom Point are used for it, I get that. If you can wear Armor over the Astral Skin, will this hamper other suit options like, Augment Weapon, a second application Nimble, Flight, or Underwater Breathing and Swim Speed?

What are the ramifications of changing Armor Class to be Constitution-based instead of Dexterity-based?

It’s never made much sense to me that your Armor Class is based on your Dexterity modifier, which carries the implication that you’re harder to hit because you’re somehow nimbly moving out of the way of an attack, whereas armor is about making you harder to injure (which seems right up Constitution’s alley, particularly as inanimate objects also have an AC).

I’ve been considering implementing a rule in my games that changes AC to be calculated based on your Constitution modifier instead of your Dexterity modifier, so per the title, what would be the ramifications of this?

The only thing I can currently think of is that it’d make some classes a little less MAD and making Dexterity a little less of a golden stat; I couldn’t see it affecting Unarmored Defense, Mage Armor, a sorcerer’s Draconic Resilience, since those provide alternative ways to calculate AC already, but I’m sure there’s something I’m missing somewhere.

Shouldn’t a net (or grappling) attack ignore armour / armor?

I look at grappling and net attacks and I’ve tried them extensively in game both as a player and as a DM. I wouldn’t want to unbalance the game but it’s not clear to me that in reality it would be any harder to grapple a night in plate armour with a net than it would were he unarmored.

Also the net is really completely useless as it stands. I’ve tried it for the monsters, my players have tried it as players. I’ve tried it as a player. It’s just a waste of everyone’s time… at best it causes a player to lose an action… and when you’re trying it on the tank then invariably it misses anyway with no chance of even doing damage. If you want to make a dent in the party it’s much more effective just whittling away at their hit points.

In my view it would be better (and make a lot more sense) if it ignored armour. Only DX mod should count against such attacks. I’d go further and say that the same argument could be made for any type of grappling.

Now I understand that RAW does not make this allowance but my question is – should it? Maybe this is something that should be changed. And I also understand that I’m probably going to get shut down because I’m asking for opinion, not rules interpretation… but I want to socialise this idea in some way and can’t think of a better platform.

There are many benefits to it. If grappling is effective then it will be used… which means less death in the campaign, which can lead to more Role Play.