(I found this)
This homemade design is an incredibly simple homemade air gun, using a ball valve as a firing trigger. I don't own one of these, nor do I intend to due to my dislike of ball valves, but this write-up is here to introduce a design to those who do not yet understand the operation of a basic homemade air gun. Basically, it's here to help those new to the field of Homemade Air Guns, utilized in Nerf. Feel free to play around with any concepts introduced in this write-up.
1" PVC Connector
1" PVC End Cap
2 x 1" PVC Tee
2 x 1" PVC 90º Elbow
Tubeless Tire Valve
1" PVC Ball Valve
Foam Craft Sheet (For optional breech)
1/2" Drill Bit (Size required may vary)
The operation of this gun is simple enough: Basically, air is pressurized and pumped into the air tank, which doubles as a stock, via a tire pump (separate) hooked up to the tubeless tire valve. The tire valve has a built-in one-way valve, so air can go in, but not out. At this time, the ball valve is in the closed position, so the pressure builds up. When one wants to fire, they open the ball valve, allowing the air pressure to diffuse at an incredible speed to a lower pressure gradient. Basically, it escapes out the barrel real fast, and takes the dart with it. The ball valve can then be closed again, and the tank refilled with air.
Making this homemade is as simple as it's operation: Just apply some primer to the inside of each connector or end cap, and the outside, end, of each piece of PVC pipe, then apply glue to those same areas, and push one into the other as in the diagram. Like the instructions right off the bottle of PVC glue. For the tire valve, drill a hole about 1/2" (or whatever diameter your tire valve requires) in the center of the 1" PVC end cap, apply some Plumber's Goop to the base of the Tire valve and the newly drilled hole, and push the tire valve through the end cap, going from the inside, so that it pokes out. Otherwise, everything else is straight-forward.
Also, this diagram describes an "Unknown Breech." This is optional, and can be replaced by buying a 1", 3/4", and 1/2" PVC Reducer.
Now, for the fun stuff!
Alright, I lied, it's not that fun. But it's one up from the basic Ball Valve homemade. Basically, you fill the gun with air from a tire pump, through the tire valve, as before, but now, you can open the first ball valve to allow the air to equalize into a "firing tank." Close the first valve again, and open the second to allow the air in the firing tank into the barrel, firing the gun. Close it up again, open the first valve again, and the air will equalize again, refilling the firing tank. Rinse, repeat, and you have a multiple shots from one filling.
The only thing really new on this homemade is the handle, attached to the first ball valve. Just cut a groove in a piece of PVC with a scroll saw or hacksaw, and use epoxy, Plumber's goop, or PVC glue to attach it. Nothing special or required, it's just there if you want a second, rotating handle for reloading. Remember, the entire idea behind homemade air guns is customization.