Each blast creates a shockwave that stuns Bloons at impact point and pops Bloons further out.
~ BTD6 description
Shell Shock is the third upgrade of Path 1 for the Mortar Monkey in BTD6. It allows the mortar to create secondary explosions that can stun affected bloons for 0.5 seconds that were damaged near the center of the main explosion radius and/or pop up to one layer of bloon depending on how close to the center of original explosion the bloons were. The secondary explosions will pop up to 45 bloons, just like the explosion pierce cap for previous upgrades. In total it has 45 pierce from the main explosion and 45 pierce from the shockwave.
It costs $925 on Easy, $1100 on Medium, $1190 on Hard, and $1320 on Impoppable.
As the cheapest tower that can inflict stuns on bloons, it can be used as cheap stunners of regular bloons.
The advantage of Shell Shock compared to Bloon Impact is that Shell Shock Mortars can be targeted around anywhere on the map.
The only major disadvantage of Shell Shock compared to Bloon Impact is that Shell Shock cannot affect Black Bloons without upgrading to The Big One or without gaining special black-popping grants, as well as that Shell Shock only stuns bloons for half the time that Bloon Impact would.