Just for reference, the "Armor as DR" rules are available here. As with many PF materials the writing is a little convoluted and the examples don't include any work, so it's a little messy.
Oddly, the book containing these rules is also the book containing the Gunslinger class, but it somehow misses the description of firearms. The rules are tweaked just enough that it's not obvious. So let's break out this new Defense score. Instead of AC you a Defense score equal to the sum of:
- shield bonus + shield enhancement bonus
- Dex modifier
- armor enhancement bonus
- Deflection, Luck, etc. except for Natural Armor bonuses
You also get a stackable DR equal to the sum of the following:
- Armor Bonus
- Armor Enhancement Bonus
- Proficiency Bonus (+1/+2/+3/+4 at level 5/10/15/20)
- Natural Armor
The Defense score kind of looks like a Touch AC, except for that shield thing, in fact the rules acknowledge this:
Defense is similar to touch AC in the standard Pathfinder Roleplaying Game rules, but...
However, the rules do not include any form of substitution for the Touch AC. It's kind of like Touch AC just goes away and is replaced by Defense score.
should firearms ignore all DR?
The first thing to figure out is if firearms should ignore shields. I would posit that keeping shields as part of the Defense score is actually a fair way to go. In fact, it gives the shield some extra utility not really found in the core PF game. Plus it's easy to reason that a bullet stopped by your shield hasn't really "hit" you any more than an arrow has.
The next thing is the DR "type". The RAW have two tables converting both Armor and Natural Armor to DR/something that can be bypassed. You may just want to give guns a bonus "bypass" on DR. So a firearm in "close" range would act as if it were magic. A magic firearm would act as if it were adamantine, etc.
This tweak keeps the relative balance. Gunslingers are now just as likely as everyone else to hit with their attacks, but they get to bypass Armor DR which lets them deal damage as intended. This is actually a good deal for Gunslingers as they tend to suffer in the face of DR, especially before level 5 when they get to add their Dex to damage.
If so, should this also apply to magical armors as well, or should they get some (Or all) of their DR against a non-magical firearm?
I think the above tweak handles this question as well. Because the gun simply grants a higher bypass on DR, everything shakes out congruent with the existing rules.
- Regular Firearm vs Regular Armor => Firearm wins
- Regular Firearm vs Magic Armor => Firearm loses
- Magic Firearm vs Magic Armor => Firearm wins
- Magic Firearm vs Magic Armor + Natural Armor => Firearm wins (it satisfies /adamantine)
Note that this is still relatively fair. The effect does not do as much raw damage as Two-Handed Fighter and the DR bypassing is still pretty fair. Note that when your Cavalier gets his first magical lance, he will be bypassing opponent's regular armor, that will make the Gunslinger's damage look very small indeed :)