I was looking around in my dad's basement and stumbled accross this air-cannon booby trap. I built it back when I was 11 or so. It's hilariously overcomplicated, but it actually still works.
That hardware in the middle is a trip-wire trigger mechansim made from a mouse trap and other odds and ends.
The trigger is pretty simple. The yellow rubber hose trys to pull the valve open, but a hook holds the handle in place. When the mouse trap goes off it knocks the hook away and the valve snaps open.
The trip wire's attached to the mouse trap's trigger.
Now that I've taken a second look at it, I'm thinking that with a bit of redesigning it would make a great airsoft or paintball mine.
You could make the air-chamber a lot smaller and the barrel shorter. With the judicious use of latches and leverage you could even do away with the mouse trap.
Anyway... More pics!
We only tested this on a person once that I remember; we loaded it with airsoft bb's. My friend volunteered to test it with a bunch of coats on.
Unfortunately, we loaded LOTS of bb's and one got him between the collar and mask. It just left a welt, but getting hit in the neck is pretty painfull. We decided not to risk it and left this thing in the basement after that.