Ammunition, +1, +2, or +3

From Dungeon Master's Guide, page 150. Found on Magic Item Table B.

Description
Weapon (any ammunition), uncommon (+1), rare (+2), or very rare (+3)

You have a bonus to attack and damage rolls made with this piece of magic ammunition. The bonus is determined by the rarity of the ammunition. Once it hits a target, the ammunition is no longer magical.