I'm trying to emulate cold iron and silver ammunition. What I ended up doing was using "damage bonus vs. race" and set the damage type to magical. I used 1d6 damage for arrows, 1d8 damage for bolts and 1d4 damage from bullets. What you get is the damage reduction absorbing the regular damage and then the magical damage bonus bypassing the damage reduction - not perfect, but it works for my purposes.
P.S. ShadowM, you should remove the top link in your signature - it points to the old vault.