It's always seemed strange to me that D&D 5e doesn't have a proper armour piercing mechanic; the main "armour piercing" in the game comes in the form of spells designed to bypass armour (via saving throws) or to target it (such as Heat Metal), but when it comes to weapons that should punch through armour a bit more easily we have nothing.

So I've been trying to come up with a homebrew armour piercing weapon property, and this is what I've come up with:

Armor Piercing

A weapon with this property treats a target's AC as a number of points lower (in brackets) if that target has natural, medium or heavy armour (such as chain mail, plate etc.). A versatile weapon may have two values, in which case the first is applied for one-handed attacks, and the second for two-handed attacks.

Weapons

A weapon property isn't much use without some weapons to put it on, right? With that in mind I decided to modify some of the weapons in the base game to include it; in many cases this means that weapons with armour piercing deal less damage, with the intention being that this is compensated for by the increased chance to "hit" (damage at all) a target with armour.

This means that many ranged weapons have more similar damage rolls, but differ by armour piercing ability, which IMO makes sense, as in a simple system there isn't much practical difference between a wound from an arrow, crossbow bolt or bullet, the real difference is that some are harder to prevent than others.

I haven't done the maths, but if the intention is to reduce the basic damage so that everything balances out then all you're doing is adding additional complexity while keeping everything the same which serves no purpose. At later levels, the +1 or +2 to hit (same effect as giving negative AC points) will drive damage up much higher as characters have magic versions of these weapons, high ability score modifiers, and effects such as [Tooltip Not Found] which make the damage roll much less relevant, while keeping the benefit to hitting.

I would suggest that armour types are better represented as follows. If a creature is wearing the following armours give it damage reduction:

• Chainmail: Reduces slashing damage taken by 1.
• Splint Mail: Reduces slashing damage taken by 1.
• Half Plate: Reduces slashing and piercing damage taken by 1.
• Full Plate: Reduces slashing and piercing damage taken by 2, and bludgeoning damage by 1.

This is much easier to keep track of, doesn't require weapons to change, and is in line with the Heavy Armor Master feat. I am always looking for ways to reward characters that wear armour (I hate that maxed DEX and studded leather is nearly as good as full plate).

I'm more of a fan of Sanvael's option, as it allows armour to defend against specific types of damage, and makes it something for the player/DM running that character to worry about rather than a constant interaction with every attack including "do they have armour on?".

It also adds a lovely mechanic where just being hard to hit won't help you if you do get hit, whilst actually wearing armour means not only are you harder to hit, but if it does get through, some damage is lessened. I'd keep the AC modifiers as well, but this rewards opting to wear unstealthy armour instead of just dodging things!

I thought that Adamantine ammo & weapons were meant to be the Armor Piercing options this edition.

Quote from Sanvael>>

I haven't done the maths, but if the intention is to reduce the basic damage so that everything balances out then all you're doing is adding additional complexity while keeping everything the same which serves no purpose.

It's not intended to balance out directly; the piercing weapons should be better against armoured targets, and less so against unarmoured/lightly armoured, i.e- against an unarmoured foe a bow, crossbow and pistol will be broadly the same, but against armoured targets this changes. 

I would suggest that armour types are better represented as follows. If a creature is wearing the following armours give it damage reduction:

• Chainmail: Reduces slashing damage taken by 1.
• Splint Mail: Reduces slashing damage taken by 1.
• Half Plate: Reduces slashing and piercing damage taken by 1.
• Full Plate: Reduces slashing and piercing damage taken by 2, and bludgeoning damage by 1.

This is much easier to keep track of, doesn't require weapons to change, and is in line with the Heavy Armor Master feat. I am always looking for ways to reward characters that wear armour (I hate that maxed DEX and studded leather is nearly as good as full plate).

While I like that idea in general, in practice this will only further penalise those who don't wear armour, e.g- Barbarians and Monks.

There are also all kinds of naturally armored creatures. So you'd need to look at each monster individually and determine which category their scales or shell or whatever is equivalent to. I feel like any way you try to tackle this you're going to get a lot of complexity with very little return in the form of actually changing outcomes.

Regarding penalising those who don't wear armour - that's kind of fair. If your whole thing is "I can dodge bullets", then you'd better be good at it (high Dex) or you might as well wear some armour!