Heres a great way. Might crash your server tho, depending on how much projectile spam you can handle
On activate/self/setvariable/Missle/1
On activate/self/ifvariable/missle/equals 1
Variabletrue/self/ifvariable/missle/equals 1
Variabletrue/self/spawnprojectile/(Your choice)
Enjoy, you need variables events mod to do this tho.