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!
Conveniently, 1-inch PVC fits quite nicely inside 1 1/4-inch PVC. These sizes of PVC make up the barrel and piston sections of this water gun.
From one 10-foot length of PVC of each of the above mentioned sizes, you can make four 30-inch water guns or six 20-inch guns. The 30-inch guns are best for older kids and adults, whereas the 20-inch guns are easier for little kids to handle. Either way, the design is exactly the same.
This design is pretty simple and easy to mass produce. However, certain parts may be tricky to find. I had to check a couple of hardware stores before I found all the pieces to make this work.
In addition to the pipe, for each gun you will need the following:
I say "not neoprene" because common neoprene washers would not work for me. When I put pressure on the piston, water would just shoot out the back of the barrel. One of my local hardware stores carried real rubber washers which were a little stiffer than the neoprene ones, and these worked much better.
There are a couple of options for the 90-degree elbow, which will become the handle. I had plenty of 1/2" PVC scrap, so I got some 1" by 1/2" 90-degree elbows, and attached a small piece of 1/2" PVC to make the handle. You could use a regular elbow, or a cap instead, but I like the look and feel of the 1/2" handle.