Update! I've got a newer, upgraded water gun instructable right here!
It's a much better design, and easier to make. The original content of this instructable follows:
Recently I found myself needing a whole bunch of water guns. I didn't want to spend a lot of money, and I wanted ones that would be durable and last a long time. I figured my best bet was to just make my own.
With the basic idea of a piston/barrel-type water gun in mind, I wandered around the hardware store for a while until I came up with the following design.
I made a lot of these, and individually the guns ended up costing around $5. It'll cost you more or less depending on a number of things, of course.
They are tons of fun, and we get them out on a regular basis for family water fights. Thanks for looking. I hope you find this useful!