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.

I began by looking at a few places online, and right here on instructables for some ideas. I found the water mortar (part one and part two), and the waterzooka instructables very helpful.

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!

Step 1: Supplies

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:

One 1 1/4" PVC end cap
One 3/4" PVC plug
One 1 1/2" rubber washer, with inside diameter near 1/4" (and it must be real rubber, not neoprene)
One 1 1/4" fender washer, with inside diameter near 1/4"
One 1" by 1/2" 90-degree elbow, or similar (see below)
One 3/4" #12 pan head screw (or something similar that works)

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.

Step 2: Barrel

Begin by cutting your barrel and piston pieces. Whatever length you choose, simply cut the barrel and piston pieces exactly the same length.

The 1 1/4-inch end cap needs a hole drilled in the end to become the nozzle. I prefer 1/4" or 3/16". To center the hole, I predrilled a smaller hole from the inside of the cap. (The end of my drill fit nicely into the cap, which centered this little pilot hole almost automatically.)

Use PVC cement to glue the cap onto the end of the barrel piece. I used sandpaper to knock off the sharp edge of the other end of the barrel.

Step 3: Piston Plug

One of the biggest tricks with this design was figuring out how to quickly and easily seal the end of the 1-inch PVC piston without adding something that was bigger than the pipe itself. The answer was to shave down a 3/4" PVC plug.

This plug is kind of an oddball fitting, so you may have to search a bit for these. They are the same outside diameter as 3/4-inch PVC.

Use a utility knife to carefully shave down the plug so it will fit into the 1" pipe. PVC cement will act as a lubricant to help get the plug in place. Drill a hole in the end of the plug that will tightly fit the screw which will hold the washers in place.

Step 4: Piston Squeegee

The washers are screwed into place at the end of the piston, as shown. I wrapped the screw with teflon tape.

Step 5: Piston Handle

A handle was added to the opposite end of the piston. There are a number of options for this part, as I mentioned before.

Smear a bit of vaseline or other waterproof lubricant inside the barrel before putting the piston in place. This will keep things sliding smoothly.

Step 6: Make a Bunch!

Now make a bunch, and have some crazy water fights with your kids (and all the kids in the neighborhood). When everyone sees how much fun they are, they'll ask you to make them some. Feel free to charge them more than $5 apiece!

To fill the water gun, put the nozzle end under water and draw the piston back to suck water into the barrel. It's important not to draw it back too fast, or to try to shoot the water out too fast either. It takes a couple of tries to figure out the correct amount of pressure to use.

One last thing... don't store the water guns with the pistons inside the barrels. You don't want the rubber washers to take on any memory.

Good luck!