I used to argue with one of my old DM's about this. Previous to the errata that showed my stance was the correct one, I had brought up to my DM that the intent was for a magic bow to do full damage to a creature even the ammunition was not magical because of the way damage resistance is worded and the way weapons are shown on the equipment charts. Despite arrows doing the damage when shot, the PHB doesn't have damage listed for them. It is listed by bow. The weapon being used by the character is a bow, not an arrow. And if the bow is magical then a magical weapon dealt damage when the arrow hit, not the arrow itself, at least from a rules standpoint.

I know it was nitpicking, but it seemed obvious to me that the intent was the bow was doing the damage since it had a damage entry and arrows did not.

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