I'd avoid damage reduction for armor of any type in D&D. Not because it's not a good idea or workable system, it just doesn't work well for the system as it's currently designed.
First off, there are a lot of kinds of natural armor. For a simple example, though, let's start with bone.
A minotaur skeleton has an AC of 12 and it's listed as natural armor. A regular skeleton is listed as AC 13 with armor scraps, so presumably it's an AC 12 with no scraps.
Logically, piercing weapons (a rapier, arrows, etc.) would be practically useless. But against bludgeoning weapons, the natural armor is pretty useless. Instead of messing with AC, they have a damage vulnerability to bludgeoning weapons. I think they should have resistance to piercing as well, but they don't in the current system. Slashing weapons are a wash for me, because most of the slashing weapons are really wielded as chopping weapons (which doesn't exist in D&D).
So then look at an killer whale. It also has a natural AC of 12. A baleen whale would be a better comparison, but we don't have stats for that. But, slashing and bludgeoning weapons are largely ineffective against a creature with such thick skin and a layer of protective blubber. But piercing weapons like a harpoon can take down a creature as large as the ship hunting it.
I guess basically what I'm saying is that 'natural armor' is too broad a category. It's better to use the existing resistance and vulnerability rules to account for special considerations on a creature-by-creature basis than to try to make a blanket rule.
As far as magic missiles taking down a wall? That's actually covered in the rules where is simply says - if you don't think the attack is strong enough to actually damage the object, then it doesn't. I do like the 3e hardness rules (and may still use those) because they simply eliminated any damage below a threshold, rather than dividing it by two. In which case you'd have to have a very powerful magic missile to damage the wall.
Ilbranteloth
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