On page 562 of the Core Rulebook:
Overcoming DR: Damage reduction may be overcome by special materials, magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment.
Ammunition fired from a projectile weapon with an enhancement bonus of +1 or higher is treated as a magic weapon for the purpose of overcoming damage reduction. Similarly, ammunition fired from a projectile weapon with an alignment gains the alignment of that projectile weapon (in addition to any alignment it may already have).
Weapons with an enhancement bonus of +3 or greater can ignore some types of damage reduction, regardless of their actual material or alignment. The following table shows what type of enhancement bonus is needed to overcome some common types of damage reduction.
DR Type Weapon Enhancement Bonus Equivalent
Cold iron/silver +3
* Note that this does not give the ability to ignore hardness, like an actual adamantine weapon does