I was bored one day so I decided to make a gun to shoot my brothers with. I needed a projectile that was soft, readily available and inexpensive. The answer: marshmallows! A marshmallow gun is usually very simple: a tube that you put a marshmallow in. You blow in one end and the marshmallow flies out the other. I wanted to make something more complex. I got a one-way air valve, some PVC fittings and a ball valve and created...(drum roll)...the pump-action marshmallow gun!

This is my first instructable so feel free to leave comments and tips.

Step 1: Materials

Materials:
4 1/2" PVC 90 degree joints
1 1/2" PVC coupling
1 1/2" PVC ball valve
(I think you have discovered by now that all the PVC components are 1/2".)
1 PVC tee
1 PVC one-way valve (Look for it under the name "check valve")
1 PVC end cap
Assorted lengths of PVC pipe (1/2", of course)
Dowel that will fit into the pipe
Rubber stopper

Step 2: Assemble the Stock

The stock is made of all four angle joints and the coupling. Fit all the pieces together but don't cement then yet. We will need to adjust this later. The picture probably explains better that I could put into words. After all, a picture is worth a thousand words.
(The stock also doubles as an air tank.)

Step 3: Assemble the Barrel

Now we come to the barrel. For the barrel, take the ball valve and cut a short length of PVC to connect the valve to the coupling of the stock. Cut another short length of PVC to connect the other side of the valve to the tee. On the other side of the tee, put a long piece of PVC. This is the actual barrel. Make sure you get it to the right length for you.
Again, don't cement the pieces together! (I made that mistake.)
Note: I attached a short bit of wood to my ball valve to make it easier to turn. That is what the long wood thing is in the picture.

Step 4: Assemble the Pump and Plunger

For the pump, use a piece of PVC about half the length of your barrel. This is your pump tube. Attach the one-way valve to the end of that. Cut a short piece of PVC to connect the other side of the one-way valve with the last angle joint of the stock assembly. Make sure the one-way valve is pointing the right way. It should let air in to the stock assembly. You can test by blowing into it.
Now for the plunger: Take a dowel that is about three inches longer than your pump tube. Get your rubber stopper and make a hole in it. Put a screw through the hole and screw the stopper to the end of the dowel. The stopper should be able to fit loosely in the pump tube. Now tighten the screw and the stopper gets wider. Tighten it until it is just able to slide with effort along the pump tube. You could probably use petroleum jelly or Vaseline to lubricate the plunger and increase the seal but I had none of this so I had to do without it. Another good idea is to put a screw or nail in the top of the dowel to stop it from sliding all the way down the pump tube and getting stuck in the one-way valve. It is a pain to get it out when it gets stuck there.
Important note: DO NOT put WD-40 on the plunger or in the tube! While it sounds like it would lubricate it nicely in reality it causes it to seize up after a time.
Now that you have both components, it is now time to drill the holes. Pull the plunger out of the tube until it is an inch or so from the end. Then drill four holes in the pipe 1/2" below the bottom edge of the plunger in the direction of the stock assembly. These holes allow air to enter the tube without you having to pull the plunger all the way out of the tube.

Step 5: Test Fit and Cement

Hey, that rhymed!
Make sure all the parts are moving correctly and then cement it together.
Don't cement the barrel onto the tee just yet.
Note: Don't sniff the glue fumes. It smells terrible!

Step 6: Use the Gun

To use: Get a small length of PVC. Plug one end into the tee. Fill the pipe with marshmallows. Put the end cap on. Pump the plunger as many times as needed. Then, very quickly, open the ball valve. Hopefully, a marshmallow will shoot out and the next one in the pipe will fall down to take it's place. I have not tested the gun in this mode of operation yet.
The gun also works as a Nerf-dart launcher. Simply remove the barrel from the tee (I told you not to cement the barrel on for a reason!) and slip in the dart. Replace the barrel, pump the gun and shoot. To get good speed, you need to pump it about four times. With enough pumping, you can achieve frightening speeds!
It only works with the suction-cup type Nerf bullets.

If you make this, please post pictures!

I know there is room for improvement in this gun, so if you make any modifications or changes in design please tell me about it (and post pictures).
Thanks for looking!
And please, please, PLEASE don't shoot anyone's eye out! :-)