I have a suggestion (and given I am a moderator on two watergun forums, it might be worth listening to).
What you should do is a piston style cannon
like this. Basically, you have a long pipe divided into two chambers by a piston (With O-rings of course), where one of these is full of water with a ball valve as a trigger (Unlike in the spudding world, the ball valve is the best valve in the watergun world because of it's clean flow), and the other chamber is full of your pressurized air.
You should only need a ball valve, quick disconnect, and a length of dowel to push the piston back down again. Filling is done via the ball valve nozzle.
Gets huge range and output. It won't be particularly easy to make the piston, but it is simply the best way of doing a watergun of this variety.