One of my players has a +1 shortbow, and recently found some +1 arrows. If he fires a +1 arrow from a +1 bow, will the bonus to attack and damage be +1, or +2?
According to 5e designer Jeremy Crawford, "Bonuses stack, unless a feature or item tells you otherwise." Therefore, firing a +1 arrow from a +1 shortbow results in a +2 to the attack and damage rolls. The tweet exchange occurred on November 28 - December 4, 2014 and is below.
@JeremyECrawford already have a question. Ranger with Arrow +1 and bow +2. Do the bonuses stack, or take the higher?
@rwallace8665 Bonuses stack, unless a feature or item tells you otherwise. And remember that magic ammunition loses its magic when it hits.
@JeremyECrawford thought so, but thanks for verifying. I'm going to guess that rolling +1 ammo on the random [DMG] table means one, not a quiver?
@rwallace8665 That's correct. A generous DM could certainly turn it into a quiver!
The Attack Rolls section in PHB says (emphasis mine):
To make an attack roll, roll a d20 and add the appropriate modifiers.
While in the section of Damage Rolls one reads (emphasis mine):
Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.
Hence, by a strict reading of the rules you can stack any modifiers you have both for attack and for damage rolls. For the former case, the word appropriate leads to include any kind of bonuses the weapon (or ammo) gives, unless the description specify the exceptions.
Do the AC bonuses from a ring of protection and bracers of defense stack? Yes. In general, bonuses stack [...]