First, name the bot spawn as something. For example, "bot1"
Now, the events on the barrel:
Onprojectilehit, self, spawnexplosion, [whicheverexplosionyouwant]
Onprojectilehit, "bot1", botAddVelocity, (make the bot get blown away from the barrel)
Onprojectilehit, "bot1", botKill
Onprojectilehit, "bot1", setvehicle, [NONE]
Try this, hope it works.