This one’s a little bit of a doozy. The most recent FAQ describes how ammunition bypasses DR, and the answer is not how most of us have been running it:
Magic Ranged Weapons and Ammunition: When a ranged weapon shares its enhancement bonus with its ammunition, does this count as “true” enhancement bonus or more like a temporary bonus like greater magic weapon? In other words, does the shared enhancement bonus allow the arrow to bypass damage reduction as if it was cold iron, silver, adamantine, and aligned?
No, other than the ways indicated in the Core Rulebook (if the ranged weapon is at least +1, they count as magic, and if the ranged weapon is aligned they count as that alignment as well) the enhancement bonus granted to ammunition from the ranged weapon doesn’t help them overcome the other types of damage reduction. Archers and other such characters can buy various sorts of ammunition or ammunition with a high enhancement bonus to overcome the various types of damage reduction.
If you have a +4 bow, that bow does not help your mundane arrows cut through DR that can be bypassed by cold iron, silver, or adamantine. That suggests that archers should invest in either the Clustered Shots feat, or invest in a multitude of arrows. One common approach is to buy durable arrows made from adamantine, which are 61 gold pieces each but can be collected afterward–assuming that you haven’t fired them off into the abyss somewhere. Remember that slashing and piercing damage done with silver ammunition takes a -1 penalty to damage; when you’re looking to bypass DR/silver, consider buying blunt arrows made from alchemical silver.
Also remember that this rule applies to other weapon types that use ammunition; gunslingers’ firearms, for example. This may actually be the more significant change, since gunslingers have fewer tools for working around this ruling. Clustered Shots still applies, but durable ammunition is not available for a firearm.
Linda Zayas-Palmer has posted the “rebuild” allowance for this FAQ. Players who have a bow with an enhancement bonus of +2 or higher may convert the existing enhancement bonus into an equivalent number of levels worth of special abilities at no cost. Note that this does not allow you to sell back an enhancement bonus for gold–it’s just a straight conversion.
If you have a +1 composite longbow, you can’t convert it into anything. It doesn’t qualify for the rebuild.