Barbed Darts is the first upgrade of Path 2 for the Monkey Sub in Bloons TD 6. It increases pierce of each main dart from 2 to 5. For Airburst Darts, darts still split on impact but each individual airburst dart receives +3 pierce. For Bloontonium Reactors, pierce of the damaging radiation zone increases from 50 to 70, and gives +20 pierce for the decamo pulses of Submerge and Support and above.
Barbed Darts will produce more pierce potential overall compared to adding Twin Guns, but for the same cost it doesn't have as much early-game value in comparison. This is because a majority of the issues in early-game come from the faster bloons rather than grouped bloons. However, this upgrade can come in handy when extra dart pierce is necessary.
This upgrade increases pierce, allowing for better popping power. It is usually just a stepping stone to higher upgrades.
There are some uses out of the 0-1-2 for advancing against grouped rounds. However, in general cases, grouped bloons aren't always a problem and 2-0-2 is better for general cases.
Mostly implemented crosspath interactions with non-dart attacks.
Heat-Tipped Darts crosspath now gives submerged Bloontonium Reactors with extra pierce.
Pierce bonus for submerged Bloontonium Reactors and Energizers decreased (+20% → additive +30; 84 → 70 for Reactors, and 1200 → 1030 for Energizers)
Energizer pierce reduction is undocumented in Ninja Kiwi's Version 23.0 patch notes.
Similar types of camo reveal like Shimmer/Signal Flare/Embrittlement are all pierce limited, but the Submerge sub has practically infinite pierce while also being extremely cheap with a very fast rate and a T4 that is far better at camo removal than those other towers can reach. Giving this upgrade a more reasonable pierce value brings these more in line and allows for the pierce crosspath to have some potential meaning.
~ Ninja Kiwi, referring to Upper Path 1 Sub
Barbed Darts now adds a limited pierce bonus for Upper Path 1 Sub decamo pulses instead of leaving them at infinite pierce.