Bloon Buster

Bloon Buster increases the damage of each Mortar shell from 2 to 3. The extra damage lets each explosion strip more layers at once and helps it get through the explosion-immune layers of rainbow bloons. It also strengthens bottom-path burns:

• Shattering Shells: burn ticks go from 5 to 8, and the blimp-only burn from 20 to 25.
• Blooncineration: blimp burn goes from 100 to 150 per tick, and walls of fire from 1 to 3 damage.

When to buy it

Bloon Buster sees some use in the early game of races, but its main job is as a crosspath. It is the best crosspath for middle-path Mortars:

• 2-3-0, 2-4-0, 2-5-0: Heavy Shells, Artillery Battery and Pop and Awe all deal more damage per shell.

For Blooncineration, 2-0-5 is preferred on maps with one entrance per round, where the extra burn damage on blimps pays off.

It is also the step toward Shell Shock, The Big One and The Biggest One.

Is it worth it?

Yes, as a crosspath it is one of the best $500 upgrades on the Mortar. Always pair it with Bigger Blast, since you need that first anyway.