Powerful Darts is the first upgrade in path 2 for Dartling Gun in BTD5, which allows the Dartling Gun's darts pop 3 bloons each at greater projectile speed. It costs $510 on Easy, $600 on Medium, $650 on Hard, and $720 on Impoppable. The Dartling Gun is much more powerful with it. This upgrade also gives a slight dart rate increase for the Dartling Gun, just like the Missile Launcher upgrade on the Bomb Tower.
There used to be a bug in Bloons TD Battles Mobile where the Powerful Darts description was not updated to match the existing buffs that were done in Version 4.3.1. This has since been fixed in Version 5.0.