Armor Class in a D&D 5e Swashbuckling Setting

Watch out! It's the pernicious influence of The Princess Bride!

This has been nagging at me for a couple of days, so I need to surgically remove it from my brain. 

We all know that one of the challenges of changing the setting of D&D from a quasi-medieval world to a quasi-early modern world is Armor Class. In D&D proper, how hard it is for your foes to hit a character (one’s Armor Class or AC) is determined almost entirely by what armor the character is wearing. Characters prancing around in silk shirts or even leather jerkins are at a severe disadvantage compared to characters in plate mail. Heavy armor, however, just doesn’t fit the aesthetic of sword-and-cape adventure, so how does one incentivize the swashbuckling style in D&D?

The most obvious incentive is also the historical one: firearms. If firearms are plentiful – and if they pierce armor as they do in real life – then players and characters will value the un-pierceable AC bonuses from high Dexterity scores, leading them to choose light armor that does not interfere with those bonuses over the dubious protection of heavy armor that negates Dex bonuses. I do not believe that firearms as presented in the 5e DMG have any armor-piercing rules, but this would be an easy fix.

This is still going to lead to relatively low Armor Classes, making even high-level characters vulnerable to low-level mooks. One way of handling this would be to include more magical protection items as the campaign progresses, allowing characters to accumulate magical amulets, bracers, rings, and silk shirts that increase their AC. This might be the simplest option for the outright fantasy swashbuckling world I proposed in my last post, but it’s less useful for low-magic or historical settings.

For those more realistic settings, I have two ideas: add characters’ proficiency bonuses to their Armor Classes (a significant change to the rules) or make the Parry action implicit in the Defensive Duelist feat and several NPC statistics into a combat action available to everyone (a much less significant rule change). Adding the proficiency bonus to AC has the advantages of being easy to track (it modifies the base AC in all situations) and not requiring an action to use (thereby not changing the basic action economy of the game). The Parry option, on the other hand, just feels cooler.

For those without access to the optional feats listed in the Players Handbook, let’s unpack the Parry action from the Knight statistics on p. 54 of the DM’s Basic Rules:

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Parry. The knight adds 2 to its AC against one melee attack that would hit it. To do so, the knight must see the attacker and be wielding a melee weapon.

That “2 to its AC” is equal to the knight’s proficiency bonus, so in other words (and trying not to duplicate the wording of the PHB text):

Parry. You can attempt to parry when a hostile creature scores an attack against you. To make the parry, you use your reaction to add your proficiency bonus to your AC, possibly turning aside the hit. To perform a parry, you must be able to see the attacker and be wielding a melee weapon.

I like this idea because it invokes the cut-and-thrust seesaw of swashbuckling fencing without over-complicating combat or changing the rules too far from normal D&D 5e. It strikes a good balance between the static Parry score of Savage Worlds and the complex choreography of Honor + Intrigue.

Anyway, that’s just something I needed to get out of my head. Does anyone else have any ideas for Armor Class in the context of a D&D 5e swashbuckling campaign?