I'd be interested to know about this too, since I tried generating wind effects with random spawn times. I gave the Wind object a private variable named "spawn", and set the event up like this:
Every 2000+Wind('spawn') milliseconds ---> Wind: Set 'spawn' to random(5000)
System: Create object "Wind" at position X,Y
But it didn't work :( It calculated the random figure for 'spawn' once, and then stuck to that figure for the entire playthrough. Different spawn speed for each playthrough, mind you, so it sort of worked, slightly.
I'm not sure why that doesn't work though. If it did, I imagine your shooting behaviour could work with a similar setup.