The problem with armor as DR is that the scale of damage in D&D gets pretty extreme when facing large monsters.  Classic example are Golems - Stone do 3-24, Clay 3-30, and Iron 4-40 and giants aren't much better. Even the best armor would mitigate only 1/3 of the damage, you'd be much better off if you got hit 30% less  

Similarly, it almost works too well against weapons. A dagger will never hurt someone who had armor of DR4 or better. Maybe that's realistic, but I don't think it makes a good game.

Hackmaster 5e is a D&D derived system, they have Armor as DR, but you roll both attack and defense and damage dice explode, so you get around the "Dagger can't hurt anyone" problem.

Mongoose d20 Conan had Armor as DR, but weapons did slightly more damage and weapons had an Armor Piercing value.

Both worked well, so it definitely can be done.

The main benefit of AC is that there's one less bit of math every round. That's nothing to scoff at when slug fest fights that drag on forever at one of the things I hate most in RPGs.

Armor as DR, because that's what armor generally does. I've always thought AC was a confusing term and a backwards mechanic, though now I understand the historical origin and intention for abstraction.

I like explicit literal combat systems, so I generally stick to TFT & GURPS. TFT had armor as DR. GURPS adds in things that are "like AC" at least in that they reduce the chances of being hit at all, but in ways that make detailed literal sense to me - they add active defenses (parry, block, dodge, etc) and chances that a shield gets in the way of an attack.

The damage reduction system used in the Lejendary Adventure game works really well. D&D really ought to just steal it wholesale. It used d20 for almost all damage rolls with weapons, which fits the fetish for using d20 for just about everything. Of course, the higher damage base would mean needing to scale up hit points, but since D&D has already brought hit point inflation to absurd levels, that's already taken care of.

Average human health is 20, so by having all weapons use a d20, it emphasizes that they all are potentially lethal. Different weapons have different minimum harm. If the d20 roll is less than the minimum, the minimum is used. Damage "explodes" on a natural 20, so big harm is possible even against high armor values. Criticals on the hit roll indicate the weapon harm bypasses the armor's protection, which is a really neat way of doing criticals. And a "super critical" calls for automatic maximum harm (the die is still rolled to determine whether or not damage "explodes"). It all adds up to a highly effective armor absorb system where some weapons are just plain better at hurting someone armored, but even the weaker weapons have a fair chance at doing a lot of damage to even someone with a very high armor rating.

For me the two systems for Armour I liked best were the Cyberpunk 2.0.2.0's Stopping Power and Fantasy Age's system.

And right now, I'm using armour as DR in my D&D 5e, with Proficiency added to certain classes to signify training.  Fighter types (all of them), the Rogue, a Valor Bard, any Cleric that can use Martial and/or Heavy Armour.  Debating the Blade Pact Warlock, but as we don't have one...

The biggest issue we've had with it, is that the Fighting Men with their non-scaling damage fail to do damage, so I've given them the same scaling that Cantrips get.  That's worked out well (Fighter types, and to clarify that means Fighters, Paladins, Rangers and Barbarians) so far.

Also, certain of energy attacks go right through, things like Thunder, Cold, Fire.  Force damage is actually effected, although I'm waffling on Magic Missile, but it's still a work in progress.

If you have Armour as DR, you need a way to avoid the armour, too. Daggers have killed more people in armour than almost any other weapon, but not because they're more armour-piercing than swords:D! It's because they stab through the chinks in the armour.
In Mythras, for example, you get that by using the Ignore Armour Special Effect on a critical attack:).

Conversely, trying to avoid the armour is what you do all the time with the default roll in AC systems. So the same attack to unprotected parts would signify an increase in damage, or the damage going straight to Constitution in systems where you have an HP/Con split, but it would not reduce the AC, because the attack isn't any easier;)!

The Conan d20 system was pretty good, it sounds a bit fiddly but as Conan d20 is very low magic whole swathes of standard 3.5ed complexity is cut out so it's not so bad in practice. It goes like this:
-Armor is DR but you can make a finesse attack (forget the exact term, called shot to the weak point or whatever) that makes you treat the DR as AC for that one attack and uses dex rather than strength.
-Weapons have an armor piercing value that you add your strength modifier to, if this value is equal to or greater than the armor's DR then the DR is halved.
-You have two way to stop incoming attacks: parry (modified by strength) and dodge (modified by dex) with different classes gaining bonuses to parry and dodge at different rates. To parry you need a weapon to parry with and you can't parry missile attacks but parry is pretty sweet as it allows you to use strength to give you bonuses to hit, to damage, to get through DR AND to avoid getting hit. Makes sense to have strength in a Conan game be a damn good stat.

Other nifty little bits:
-Very low massive damage threshold (trigger a save or die if you hit someone for more than 10 damage) so just getting in one solid hit gives you a chance to take someone down and bypass their HP.
-Very person after the first who attacks you gets a cumulative +1 to hit so getting ganged up on is dangerous.

Quote from: Daztur;933849The main benefit of AC is that there's one less bit of math every round. That's nothing to scoff at when slug fest fights that drag on forever at one of the things I hate most in RPGs.

Very true and this was one reason why in my own RPG I limited the options to keep things from dragging out. You could roll to dodge and deflect, or attack and dodge or deflect. But not all three. Mainly because you also each round had to declare where you were attacking and where defending. You could also throw everything into just one and get a bonus. So only a max of three rolls to resolve an attack as there was no damage roll.

I also like the original Albedo's system which was a single attack roll and then a damage roll if the attack hit. Armour could both stop the attack penetrating and distribute damage to mitigate if it stopped the penetration.

So say my diplomat is in standard ConFed flak armour which has penetration resistance 8 and impact distribution 3 and just got shot in the chest with a standard ILR assault rifle using 8mm rounds at med range. The ILR shooter needs to get a result of 9 or better after mods to penetrate the vest. If they got a 5 then unarmoured that would have been a light wound and the PC is stunned. But with armour the result is reduced to no wound and the PC is just staggered. If the shooter got a 9 then that goes right through the vest and my poor diplomat just took a massive chest wound, and was knocked down and out of it 6 rounds, AND he is bleeding out fast. If the shooter got an 11 then with a chest hit - sorry - Im rolling up a new character.

All that with just 2 rolls.

Quote from: JeremyR;933841The problem with armor as DR is that the scale of damage in D&D gets pretty extreme when facing large monsters.  Classic example are Golems - Stone do 3-24, Clay 3-30, and Iron 4-40 and giants aren't much better. Even the best armor would mitigate only 1/3 of the damage, you'd be much better off if you got hit 30% less  

Similarly, it almost works too well against weapons. A dagger will never hurt someone who had armor of DR4 or better. Maybe that's realistic, but I don't think it makes a good game.

I agree that armor as DR means that it has more of an effect against low-damage attacks, and has little effect against high damage. If you are a knight in full plate, you can take on many poorly-armed peasants or goblins, and they have little chance unless they tackle you.

However, I think it's nonsense to say that this doesn't make a good game. It works great in tons of games from RuneQuest to GURPS to Hero System. Why wouldn't it do so?

Quote from: Daztur;933849The main benefit of AC is that there's one less bit of math every round. That's nothing to scoff at when slug fest fights that drag on forever at one of the things I hate most in RPGs.

While technically true, this is a pretty minor factor. In other games, I find that it has no effect when damaging PCs (because the GM tells damage to the player, and the player handles that calculation on their own time), and minor change for GM tracking.

The main thing that makes slug fests drag on is that there are an inordinate number of repetitive rounds. To deal with this, I prefer to just up the damage done and frequency of hits.

Quote from: jhkim;933963The main thing that makes slug fests drag on is that there are an inordinate number of repetitive rounds. To deal with this, I prefer to just up the damage done and frequency of hits.

What factor of damage to total hit points do you prefer? Like are you talking one hit kills? Or two to three good whacks?

Quote from: Daztur;933849The main benefit of AC is that there's one less bit of math every round. That's nothing to scoff at when slug fest fights that drag on forever at one of the things I hate most in RPGs.

Again, this is more of an issue with level-ascending hit points, and the related "bullet sponge" issue in computer games. Take those things out and you're just left with the possibility of a "whiff factor", like one BNG said, "rpg combat consists of waiting for your turn to miss."

But you get that with DR, too, or the thing of, "you hit but do no damage." Which gets frustrating. If I'm going to fail, I'd like to do it in one dice roll instead of two.

Earthdawn uses Armor as DR, but has made a few changes in the most recent edition based on feedback from the last 20 years of play.

- Shields now add to your Physical/Mystic Defense (target number to hit) instead of Armor rating (avoids the number-escalation game, and positions shields as "mobile cover").
- Armor DR is no long "all or nothing." Previously, if you scored 3 successes on a hit (an "Armor Defeating Hit"), you completely ignored armor in the damage calculation. This led to very "glass cannon"-like combats at high levels, where relying on armor was a very risky thing to do. Now, each additional success on the attack roll adds a flat +2 damage bonus (basically allowing "partially armor defeating hits"). There are other things you can "spend" extra successes on your attack roll on, so it is an interesting addition to the system.

Quote from: Kyle Aaron;933972Again, this is more of an issue with level-ascending hit points, and the related "bullet sponge" issue in computer games. Take those things out and you're just left with the possibility of a "whiff factor", like one BNG said, "rpg combat consists of waiting for your turn to miss."

But you get that with DR, too, or the thing of, "you hit but do no damage." Which gets frustrating. If I'm going to fail, I'd like to do it in one dice roll instead of two.

Not if you have other things going on in your combat system besides rolling to hit and rolling for damage.

Can anyone guess what I'm going to suggest?

If you were thinking maps with rules for terrain, taking cover, overwatch, etc, yes.

Gameplay can be not about just picking targets and rolling until you win or lose, but how people move so as to get into situations where they have good chances to hit and concentrate attacks, where the enemy has a hard time doing anything effective.

I like random damage reduction like in BRP big yellow or Barbarians of Lemuria. Static damage reduction opens up for trouble when you cant damage someone at all when your weapon does low base damage. Which opens up for rules patches to fix this. Random  DR feel like a nice abstraction and keeps people on edge and is a good way to entirely skip rules for Hit Locations since random DR simulates this as well.

I tend to like the D&D AC- model in practice more than theory. BoL use this (Defence) in combination with DR, and Conan D20 as well. Ideally I refer the reversed AC model where the players roll their  AC vs the enemies static attack value.