I use an airsoft M203 to fire 15mm Center Fire "Bear Bangers" in 40mm aluminum adapters
http://www.truflareusa.com/cartridges.htm
They are basically "flash bangs". In low light they are insane.
It's all legal, it's not even considered "shooting a gun". They are classed as 1.4g consumer fireworks. So the worst you might get? $65 ticket for lighting off fireworks on a non-firework holiday.
Although, I'm not sure if your local airsoft field would allow their use either
I like things that go boom
-Jamie M.