Heat tipped darts allow popping of Frozen and Lead Bloons.
~ In-game description
Heat-tipped Darts is the second upgrade of Path 2 for the Monkey Sub in Bloons TD 6. It allows the Monkey Sub to pop Frozen and Lead Bloons. When used as a crosspath for the submerged Bloontonium Reactor, the Bloontonium Reactor deals +1 damage to Leads but not extra to Frozen Bloons, the pierce of the damaging radiation zone increases from 70 to 100, and pops Lead.
This upgrade will give the Monkey Sub red paint near the front of the sub, underneath the gun, as shown in the artwork. Purchasing two of the Path 1 upgrades will give the sub a green cap and green-lensed binoculars. Purchasing two of the Path 2 upgrades will give the sub two guns, as well as yellow paint around the window on the side of the sub. Aside from simple paintwork at the fronts of the submarines, Submerge and Support's glass becomes red instead of light blue, and the radiactive pulses from Bloontonium Reactor and Energizer turn red, and Bloontonium Reactor's red paintwork surrounds around the radioactive icon.
In addition to extra pierce from Barbed Darts, all attacks can pop Frozen and Lead; since all affected attacks start off with either Sharp or Shatter, this is effectively just giving Normal damage type. Reactors and Energizers also gain +30 pierce for a total of +50 pierce.
The Heat-Tipped Darts upgrade has a rather niche use, being quite ineffective for most purposes, but is still handy for certain scenarios whenever Leads or DDTs are a threat, or on Race Events when using Bloontonium Reactor. This upgrade is mostly relegated to custom challenges or very unique challenges in which this upgrade is needed.
The Heat-Tipped Darts upgrade can be somewhat useful when going towards Ballistic Missile, especially for Round 28 and 30. In particular, a 0-2-2 Sub can solo Leads when placed in a spot with decent range.
Extra damage to Lead Bloons via Bloontonium Reactor is hardly useful because the Leads will usually fully pop by the time they leave the radioactive ring. However, the extra pierce from this upgrade is useful for allowing Bloontonium Reactor to more reliably combat dense mid-game rounds.
Mostly implemented crosspath interactions with non-dart attacks.
Heat-Tipped Darts crosspath now gives submerged Bloontonium Reactors with bonus pierce and +1 damage to Lead Bloons.
Heat-Tipped Darts now also adds even more bonus pierce for Bloontonium Reactors (84 --> 100).
[Undocumented] Reduced bonus pierce for Energizers (1200 → 1050)
The logic on a Lead crosspath not being required to give Lead popping to an attack based on something weak against Lead has never sat right, so we're finally sorting that out and making sub a little angrier.
~ Ninja Kiwi, referring to Reactor
Bloontonium Reactor now requires Heat-Tipped Darts to regain Normal damage type.