Armor Class and Defense

[Rhenny]

If a critical hit penetrates DR, what creatures are vulnerable? Can you crit. a ghost if it has DR vs. non-magical weapons? Does DR apply only vs. AC or does it apply against any physical effects like a fireball or meteor storm?

I would think that any DR could be penetrated if it is related to merely armor or natural DR. Invulnerability would not be bypassed unless the proper weapon/energy form is used. On a "20" sure...hit that ghost for more damage. Let the fireball burn the creature more (unless it has a fire invulnerability).