Armor as damage reduction

Like a lot of folks, I've been thinking for a long time about representing armor in a D&D-like system with a damage reduction mechanic (which decreases the damage done by every successful attack) rather than the usual damage prevention mechanic (which reduces the likelihood of successful attacks). That just feels right to me. It would create a distinction between evading an attack and absorbing it with your armor, and that would let you play with mechanics like armor-ignoring attacks that don't auto hit and unavoidable attacks that are mitigated by armor.

I've frequently heard that this isn't viable. That it would turn combat into a slog of slow attrition.

That sounds like a problem one could solve with spreadsheets! So I set up a bunch of formulas to basically roll attacks and damage for various scenarios in a standard AC paradigm (using Old-School Essentials as my baseline) and output the average damage per attack. Then I tried the same thing with various levels of damage reduction, and I looked for where the average damage in the first paradigm matched the average damage in the other.

The results actually pointed toward an extremely simple conversion: For every 2 points of AC bonus, give the character 1 point of damage reduction. Any AC adjustment from high or low Dexterity can stay, as can bonuses from shields. (This mechanic does still use AC, but I'd just call it "Defense" instead.)

So, if a suit of chainmail gives you AC 14 in an ascending-AC system or AC 5 in a descending-AC system, it gives you DR 2 in this armor-as-DR system. If you've also got a shield and a Dexterity modifier of +1, then your Defense (essentially your AC) is 12.

Here's a whole series of tests demonstrating the overall effects of that change in nine different scenarios. As you can see, damage-reducing armor turns out to be very slightly better for the target than hit-preventing armor as long as we're only considering attacks at the lower end of the damage scale. Once the potential damage gets higher, and we look at stuff like a black dragon's bite or a Thief's backstab, damage reduction starts to fall behind hit prevention.

Overall, I'd expect this change to make fights slightly slower at lower levels and slightly faster at higher levels, which sounds good. I might also allow damage reduction from armor to apply to auto-hit area effects like fireballs and such.

Nine scenarios aren't very many, but this looks good enough to be worth testing out for real. The question is: What kind of testing would be sufficient? Since the real question is how the mechanic would hold up as the numbers get bigger, it kind of demands trials over a wide spread of character levels. Maybe I could do some kind of "You gain a character level every time you reach a deeper dungeon level" thing. It would distort player behavior, but if that leads to more fights, it's probably better for testing anyway.