"Were you looking for the Bloons Tower Defense 5 upgrade called Sharp Shots?"
Piercing Darts is an upgrade for the Dart Monkey that appears in Bloons TD 1, Bloons TD 2, Bloons TD 3, and Bloons TD 4. This upgrade allows the Dart Monkey to throw darts that can pop 2 bloons at a time. The Dart Monkey cannot pop one bloon twice in one shot (a good example being that you cannot pierce a Green Bloon with one Piercing Dart and have it pop down to a Red Bloon). The Piercing Darts upgrade was renamed to Sharp Shots in Bloons TD 5.
The appearance of a Piercing Darts Dart Monkey is the exact same as that of an un-upgraded Dart Monkey in Bloons TD 1, Bloons TD 2, and Bloons TD 3. In Bloons TD 4, it has a little arrow on it to indicate its rank. The Bloons TD 1 upgrade button has the same appearance of the Bloons TD 2 upgrade button.