One thing that I’m undecided about is D&D N5xt’s “bounded accuracy” idea. As I mentioned a little bit ago, I’m generally not a fan of it. It strikes me as an unnecessary “solution” to a problem we have created for ourselves. (That problem being that as characters gain levels and improve Skill bonuses and To-Hit bonuses, the creatures they face have higher AC and the challenges we give them have higher DCs, so it’s all a wash.) We created it when we stopped basing the mechanics off the world (that’s a really difficult cliff to climb, so DC 18) and started basing the world off the mechanics (characters at this level will have a +4 to climb, so for this to be a challenge is needs a DC 18). If you stop doing that, if you let characters encounter a world that has both trivial and impossible obstacles, then the fact that they get higher bonuses matters.
That being said… while we have a general notion of what DCs mean in terms of skill and talent and success, it seems to me that we don’t have anything similar for modeling AC and to-hit bonuses. This is particularly meaningful to me because I think combat may be the one place where bounded accuracy could make sense. I’m not convinced it does make sense, but it could. With skill checks, that cliff will always be a DC18 cliff, but if it’s windy, rainy, icy, and so on you might take penalties to your Climb check, and so having higher and higher bonuses is meaningful because not only can you succeed at Really Hard Things, but you can succeed even in non-ideal conditions. How can the same things translate to combat?
D&D 3.X and Pathfinder both have a bunch of modifiers which are used to calculate AC. These can include Size, Dodge, Dexterity, Shield, Armor, Natural Armor, Deflection, and Luck (there are other bonuses, such as Competence and sacred, but that aren’t normally applied to AC). Most of these are generally self-explanatory (a Size bonus to AC means it’s a small target, Natural Armor means it has a tough skin, and Deflection means some force is protecting the target), but what does a +5 Natural Armor bonus mean? What does a +3 Dexterity bonus mean?
Generally speaking, Armor Class is a creature’s ability to avoid being hurt. For the most part, this means avoiding being hit or, more likely, avoiding anything more than superficial injuries. “Bonuses” to AC aren’t so much bonuses as they are components of this “don’t get hurt” factor. As the bonuses add up, they determine a percentage chance that the creature will suffer an honest wound; assuming an untrained assailant, an unarmored human with average agility (AC10) is a little more likely than not to be hurt when attacked (55%), but that same human in a suite of full plate has a much smaller chance to be hurt (10%), and adding a small shield means that in almost all cases he’ll avoid actual harm (5%). Because a Natural 20 is always an automatic hit, the minimum chance to be hurt is 5%.
As a quick aside, this is why I’m in favor of confirmation rolls for critical strikes: most weapons’ threat range is simply a natural 20, and though that is at least an auto-hit the confirmation roll just has to beat the target’s AC — that is, if you could have hit the target normally, then rolling a 20 results in a critical hit, otherwise it just gives you a normal hit. At the upper end of the spectrum where you’re fighting a creature you can’t hit without the Natural 20 auto-hit, this mechanism lets you score those hits but denies you the extra damage; without confirmation rolls, higher level monsters will ignore your attacks most of the time, or be grievously* wounded, without any middle ground.
As I noted, this assumes an unskilled assailant, and that gets in to attack bonuses. Someone with average ability and no combat training will have a +0 attack bonus, and their chance to hit is based solely on their opponent’s armor (or protective spells, or ability to dodge and so on). A combat-trained character — a Warrior, Fighter, Ranger, Barbarian, or Paladin — gets a +1 bonus to hit because of their training, and overall this gives them an extra 5% change to hit their opponent. A Masterwork weapon is supposed to be finely crafted and well-balanced, and this gives the wielder another +1 (+5%) bonus to hit. As characters level up, they get more and more combat experience and that experience translates into higher bonuses to hit — they are able to find the chinks in foes armor and (more importantly) put their weapon there to score a damaging hit. Eventually even the stoutest mundane armor can be bypassed, and engaging the character unprotected would be suicide. (Though note, most groups play with a rule that a Natural 1 always misses, so the highest chance for a character to hit is 95%.)
So back to the question at hand: do we need to flatten attack and defense bonuses? In particular, if the idea behind “bounded accuracy” is to allow DMs to introduce high-level monsters and still give the party a chance of success, or present players with low-level monsters and have them still present a credible threat, then the rules of Natural 20 and Natural 1 already give us that level of “bounded accuracy.” The only question might be how high AC and high hit points interact, but Bounded Accuracy has already stated that it’s trading AC for more hit points: you can HIT high level monsters, but they can out-last you until you can deal more damage. (The Pathfinder Owlbear has AC15 and 47HP, the 5E playtest owlbear has AC13 and 110HP. A Pathfinder Troll has AC16 and 63HP, the 5E Troll has AC16 and 132HP. The 5E Troll also has double the Regeneration as the Pathfinder Troll.)
If anything, I think the Monster ACs need to be broken out better (So I can give my Troll Scale Mail if I want to) and the higher abuses of Natural Armor need to be reigned in — though most of those are creatures at the epic end of the spectrum, representing god-like beings, and I suspect they were mostly created top-down to provide a challenge for characters that are expected to have a +20 attack bonus. We need to cut that out.
*How grievous a wound is has more to do with the target’s HP than with the weapon’s damage roll, but that’s a discussion for another time.