I have tried to build an air gun SO many times, and have failed. After very much research and planning I have created a pretty decent airgun. It was planned to be built with basic tools, and you can very easily modify it to use basically anything as ammo. Nerf darts, bb's, marshmallows, beans, whatever.

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Step 1: Materials and Supplies

This projected costed about $12 not including tools.

Materials:
a short length of 1-1/4 inch pvc
a short length of 1 inch pvc
some scrap wood (I got mine for free at home depot)
a very thin sheet of craft foam
a 1-1/4 inch pvc cap
vaseline
some cord or string, i used paracord
foam board
a tube that fits your ammunition, 1/2 inch pvc for nerf darts or marsh mallows, or something about the size of a pen tube for bb's

Optional materials:
spray paint
pvc cement

Tools:
hack saw
scissors
metal file
pen
hot glue
hole punch

Step 2: First, the Design

Before you build something you should probably know how it works. This gun is basically a piston with a barrel. It can create a vacuum and pressurize air. The first picture is the concept and the second picture is sorta the build guide picture, its not the right proportions but it gives a good idea of the layout.

Green- the air chamber

Blue- the barrel and plunger

red- front and back grips

To make this tutorial easy i will show how to build each of these pieces separately, then how to assemble them.

Step 3: PVC

First thing to do is cut the pvc. You need to cut the 1-1/4 inch pvc down to about a foot long. Make the 1 inch pvc about 4 inches longer that your 1-1/4 inch.

Step 4: The Air Chamber

This part is easy. Just cut down the foam until it fits snugly in the back of the pvc cap. I attached it to the pvc after I spray painted everything else.

Step 5: The Barrel

First try to cut two circles out of the foam poster board that fit in the 1 inch pvc pipe. A way to make this easier is to cut a thin ring from scrap 1 inch pvc pipe and trace the inside onto the foam board. Then try to cut a hole as close as you can to the center of each circle that you barrel can fit through. This is very easy to do if you have a hole punch.

Then cut a length a tube your using as a barrel about an inch longer that the 1 inch pvc pipe. Then you can glue the barrel into the 1 inch pvc. make sure one side is completely flat on the bottom.

Step 6: Grips

This part is pretty straight forward. There are many ways to make a grip. It can be as simple as a wood block or a piece of pvc. I drew and cut out a grip from some scrap wood, But it did take a REALLY long time. once your two grips are made you can hot glue one to the center of the 1-1/4 inch pvc and one to the end of barrel.

Step 7: (optional) Spray Paint

Now its time to paint it if you want to. To hold up the gun I taped a 1-1/4 in pvc coupler to a piece of wood. This holds it in a way that is very fast to spray paint because you can paint the whole gun at once. IMPORTANT, Before you start painting, tape off any of the barrel that goes in the air chamber. If you have spray paint in the piston it wont work as well. You can also paint the end cap separately.

Step 8: Assembling the Piston

To get an air-tight seal you need your craft foam. Cut a length of foam about 3 inches long and a centimeter wide. Next, wrap the foam around the un-painted end of the barrel, when you do this stretch the foam out. Now Put some vaseline on the foam to keep the gun running smoothly. Then screw the air chamber onto the barrel whichever way you wrapped the foam on. This is the hardest step in the whole build and it might take a few tries.
Also the foam may deteriorate over time and you may need to replace it.

Step 9: The Stopper Cord

This cord is just to make sure nobody accidentally pulls the barrel out. First pull the barrel out as far as you can without it loosing stability or the foam coming out. Now cut a length of cord that when folded can reach the middle of the air chamber to the end of the barrel. Now glue the two ends the barrel. Lastly, Pull the middle of the cord so its taught and glue it to the air chamber. Now the cord should stop you from pulling the barrel to far out.