I've seen this question asked in previous editions, but I want to ask this question specifically for D&D 5e. Apparently there is still some debate about this. If I am a fighter with a +2 bow and I fire a nonmagical, mundane arrow at an enemy (golem, for example) that has resistance to damage from nonmagical weapons, does my attack overcome the damage resistance?
YES, a mundane arrow fired from a magical bow overcomes the resistance
In the D&D 5e Errata, this issue is addressed:
Magic Weapons (p. 140). The section ends with a new paragraph: “If a magic weapon has the ammunition property, ammunition fired from it is considered magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.”
Here are some additional reasons:
- Jeremy Crawford says so. If this isn't enough reason for you, please read on.
- In the weapons table in the PHB (p. 149), 1d8 is listed as the damage for a longbow, not for an arrow. Bows are weapons that specify that they require ammunition, and arrows are ammunition, not weapons.
- The resistance is against "bludgeoning, piercing, and slashing from nonmagical weapons" and if a +1 bow is a magical weapon, the damage bypasses resistance since it is from a magical weapon.
- A single +1 arrow is in the rarity category "uncommon", just like +1 bows. If an archer is relying on +1 arrows to be as relevant in combat against a nonmagic-weapon-resistance enemy as a melee fighter with a +1 weapon, the archer is only going to have a handful of one-off shots for their entire career if item rarities are followed from the 5e DMG. This will severely limit bow users at later levels while melee users and casters are able to do full damage. The alternative is that the DM has to give out stacks of +1 arrows, which breaks the suggested occurrence of magical items.
- According to the "Effective Hit Points Based On Resistances and Immunities" table on page 277 of the DMG, as CR increases, the effective hitpoint multiplier of a monster with immunities and resistances decreases. The claim made in the text above this table is that the change is because, as characters increase in level, they should have access to better means of bypassing the resistance or immunity. In the case of an archer, the small collection of magical arrows that they may collect in their career (again, as per rarity suggestions in the DMG) may last a couple of encounters, but doesn't represent "the means to counteract that resistance or immunity" after more than a couple of encounters with relatively common creatures at higher levels (basically anything incorporeal, many elementals, many constructs, many creatures of magical nature).
I play an archer and I would still say no, or atleast restrict the magical property to the short range bracket with the logic being that the magical bow/crossbow/sling could leave lingering enchantment on the ammo for the initial duration of its flight. I feel that being able to stack magic bonuses of ammo and weapon more than makes up for the weakness of saying the mundane ammo doesn't break damage resistance