Source: Dungeon Master's Guide

Weapon (Any Ammunition), Uncommon(+1), Rare (+2), or Very Rare (+3)

You have a bonus to attack rolls 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.

This ammunition is typically found or sold in quantities of ten or twenty pieces. Ten pieces of this ammunition are equivalent in value to a potion of the same rarity.