Extra Damage to Leads

Extra Damage to Lead is a projectile behavior characterized by dealing more damage than that tower's base damage to Lead Bloon properties per hit. It is part of the damage types family.

Extra Damage to Lead is represented on Bloons Wiki as, the upgrade icon for Heavy Shells in Bloons TD 6, because which conveniently enough deals extra damage to Leads and looks like a lead-based impact. Extra Damage to Lead is simply a subset of Extra Damage. Only three unique upgrades are known to exist in Bloons TD 6, and all are added in later updates: Heavy Shells and its later upgrades, submerged 4-2-0 Reactors and Lead to Gold.

Gwendolin's Level 17 "Heat it Up" Passive Ability also deal extra damage to Leads

All attacks require an upgrade to deal more than one damage to Lead per hit. Oddly enough, it takes at least Tier 3 upgrades to unlock this type of property.