That will only kill your own/allied caps. Use
targetFilter
numOwnerships 1
ownership "Enemy"
numObjects 1
object "CapitalShip"
numSpaces 1
space "Normal"
numConstraints 0
instead
Put it in a passive ability, which applies BuffOnWeaponFired (make sure to add the weapon class of your turret to the list, base it off Incendiary Shells) Make the applied buff either simply MakeDead or DoDamage of like 5-10k.
