We're still talking across each other here, I think.
Re: touch AC:
Even if 5e doesn't include it in the rules, the concept is still going to be present.
1. If all I'm trying to do is touch you - not damage you, just touch some part of you - but you're trying to avoid that touch, what AC am I rolling against and how does it get there?
There aren’t really any codified abilities that call for such a roll, so this would fall under the rules for improvising an action.
To perform the improvised action the DM will normally have you make an ability check. The DM will assign an appropriate difficulty class and will explain possible consequences if the attempted action fails. (...)
Most improvised actions can be resolved as simple contests.
So, it’s up to the DM how to resolve it. Making an attack roll against an AC calculated in a non-standard way is one way you might resolve that. But, as the rules for improvised actions state that you normally make an ability check for improvised actions and that most can be resolved as simple contests, I would be more inclined to call for a Dexterity vs. Dexterity contest.
2. If on the other hand I'm trying to hit you hard enough to damage you and (obviously) you're trying to avoid that hit, what AC am I rolling against and how does it get there?
The target’s AC.
The answer to 2. is obvious, as it's the AC value you use all the time in combat. But it won't be the same as the answer to 1. if you're wearing any armour; and this is the difference I'm trying to get at. If my AC is 15 and you roll 13 to hit me, you won't do any damage but chances are the DM is still going to narrate that the blow connected with me (probably my shield, as it missed by 2); where if you only roll a 3 to hit the chances are the resulting narration will involve the words "missed completely" in it somewhere.
Yes, that is all accurate.
I think the unintuitive part is the very comparison you make here between a creature and an object; a comparison that simply isn't necessary. In both cases the to-hit roll (for game mechanics purposes) is most of the time checking whether you've hit well enough and-or hard enough to cause damage.
This is where we’re disagreeing. I’m saying that when you attack a creature, you roll against a number that represents how difficult it is to hit that creature in a vulnerable spot. By default, this number is 10+Dex, representing the creatire’s ability to dodge your attacks. Armor and some abilities change this calculation, usually representing the decreased surface area of the creature that is vulnerable to attack, and potentially the increased difficulty of dodging attacks while armored. Shields and cover increase this number, representing an obstacle preventing the attacker’s blows from reaching the target.
When you attack an object, you roll against a number that represents how hard you have to hit it to do damage. This number is fixed, and does not need to be calculated.
If yes, roll damage. If no, it then becomes simply a narration question as to whether the swing missed completely, or connected but not hard enough for damage, or was blocked by a shield or by cover, or whatever.
Yes, but since the attacks represent different things in the fiction depending on whether the target is a creature or an object, the narration should reflect that difference.
But sometimes, e.g. when I've got a loaded shocking staff and all I need to do is touch you with it in order to deliver its shock, your armour and some other AC factors might not help you. In these cases, I'm rolling against a lower AC.
Most such abilities actually force the target to make a saving throw. There are no codified abilities that target a lower AC value than the target’s AC.
Mechanically, in the game it represents exactly the same thing: how high an attacker's to-hit roll has to be in order to cause damage.
Yes, which is why Barkskin shouldn’t need to explain what the minimum AC of 16 represents in the fiction. It’s a mechanical effect that sets a floor for the difficulty of the d20 roll you make to see if you can roll your weapon’s damage die. But since people seem to be struggling to come up with a satisfactory explanation of what that represents in the fiction, I am offering one. Armor makes you harder to do damage to because you can an unarmored person can be damaged by a hit anywhere on their body, while an armored person is mostly safe unless you get in the crack of their visor or under the armpit or something. Barkskin does not do what armor does. It does what bark does, which is make trees hard to damage because you gotta hit it real hard to get through. Mechanically, the latter is represented by a fixed AC value that is not affected by things like shields or cover.
