Modern day materials on fantasy armor.

[Ed Laprade]

Almost the simplest method. Though it would require a damage type (firearm or ballistic) and then an armor property (advanced or anti-ballistic).

I still will say, if you are now going to account for one type of damage and have a special type of armor for that damage type, then you should also account for the current damage types; piercing, slashing, bludgeoning.

And then are you going to use resistance, damage reduction, hardness, special AC or what mechanic?

IMO, none of that is worth it to me. Might as well use a different system than 5E if you want to go there and start tracking hit locations, lingering damage, ...

Yep. D&D armor is not, and never has been, in any way realistic. You can swing a sword at a guy in full plate armor all day, and the only effect will be a dull blade and some dings in the suit. So unless you want to bring back something like 1E's weapons vs. armor tables, it just isn't worth it. (And there's a good reason why hardly anyone ever used them.)

 

[Uchawi]

Decide on what ammunition and/or projectiles you will allow and adapt the armor. A simple approach is to have single shot (one projectile), burst shot (multiple), or explosive shot (no ammunition but channeled combustion or related chemical reactions). You can even have reactive shot where the ammo has different properties based on what it hits. After all that it will be easy to come up with armor ideas or what not to wear.

 

[Andor]

D&D armour, weapons and damage are all designed for ease of play and have little or nothing to do with realism. If you're not differentiating between a club, a crossbow bolt, and a fire bolt, why is the bullet (but only a pistol bullet, not a sling bullet) the odd man out?

There are plenty of systems with greater (Morrow Project, Albedo, etc) "accuracy" in their combat and damage systems, IME it's rarely worth the trouble.

Getting clubbed in the face might, or might not kill you. Getting stabbed in the face might or might not kill you. Getting shot in the face might, or might not kill you. Unless you are detailing the precise nature of the hit and the resulting injury in detailed medical fashion (in which case 5e is a poor system match for what you want to represent) there is little reason to model just this one specific weapon differently.

And as other have mentioned, historical plate armour is perfectly capable of defeating period firearms. There are accounts of cavaliers getting shot in the helmet with a pistol at point blank range and riding off.

If you're really keen to make guns special however, I'd have them use touch attacks vs normal armour, and then make a special armour property called "Bullet proof" which negates that and makes them roll vs normal ac like everything else.

 

[Caliban]

D&D armour, weapons and damage are all designed for ease of play and have little or nothing to do with realism. If you're not differentiating between a club, a crossbow bolt, and a fire bolt, why is the bullet (but only a pistol bullet, not a sling bullet) the odd man out?

The only reason I would consider it is because bullets are generally considered to be far more damaging than a melee weapon or bow/crossbow.

In a fantasy setting I consider them to be more on par with spells than mundane weapons. You can model this in various ways (bypassing armor, lots of damage dice, their own damage type, etc.). The more realistic you want it to be, the more complicated it becomes though.

The OP wanted something similar to modern day kevlar/ballistic plates, so I suggested Resistance - when you get hit with a bullet while wearing a kevlar vest, you still end up with heavy bruising and possibly cracked ribs - but it's a lot less than you would have otherwise. I also know that due to the way kevlar works, a piercing weapon (like a knife) tends to slide between the fibers. So I didn't think flat resistance to piercing damage would be appropriate.

So, to make things super simple (in keeping with the spirit of 5e) - make damage from firearms something akin to their own elemental damage type and have materials like kevlar give Resistance to that damage.

You could also do things like having them ignore the AC from armor (or only the ac from light and medium armor), unless they are lined with kevlar, etc.

 

[LordEntrails]

Or, to go with the idea that bullets do more damage than swords, and to keep things simple, just have pistols do 1d50 damage and rifles do 1d100.

Then you leave armor as it is, don't have to make any special rules, and you account for a bullet being able to graze you (1 HP) or blow your brains out (100 HP).

 

[TheCosmicKid]

You can swing a sword at a guy in full plate armor all day, and the only effect will be a dull blade and some dings in the suit.

I dunno, if it's a hot day, maybe he gets heatstroke.

 

[Satyrn]

I dunno, if it's a hot day, maybe he gets heatstroke.

But who succumbs first: the guy sweating in his sauna plate or the fellow doing the manual labour, overexerting himself with every swing?