SHOUTOUT TO KIPKAY FOR POSTING A VIDEO ON THIS: my youtube channel is Coolnventions
What Did I Make?
I am 16 years old and made a covert air powered bb gun, that is rapidly realizes a burst of air to propel a projectile at maximum possible velocity. I wanted something powerful, yet small and covert, so I chose a maglite flashlight because it could hold high pressures, and had a threaded back cap, which was crucial to a serviceable design
How did you make it? This was made by first disassembling the flashlight, and gutting the bulb assembly, then you are left with a empty tube. The barrel is then centered and epoxied in so that there is room left at the bottom for a piston to move. The battery cap to the flashlight is drilled, and fitted for the Shrader Valve, and then is epoxied in. Finally a piston is fabricated by stamping a thick piece of rubber. The whole thing is assembled, and is pressurized with the pump, the gun is fired by pushing the pin in the shrader valve. When the valve is pushed, a small amount of air is vented, causing a suction to pull the piston back and dump the rest of the air into the barrel.
Where did you Make it? I made this at my home, and in my garage. This was a fun summer project for me, and it turned out very nicely.
What did you learn? I learned the basic principles of Penumatic Design, and methods of achieving supersonic velocities by using physics equations, and simulations. Using the equations for drag, and velocity I was able to estimated the pressure with about 9% error to the actual thing, which was measured by a chronograph to 850 feet per second.
Materials: High Pressure Epoxy (JB Weld should work) Pipe cutter (or metal saw) Mini Mag lite Drill 4.5mm internal diameter brass tube (varying length, check next step) Shrader valve (from bike tire) Shock Pump (sold at sporting stores) Thick rubber Plumbers Tape
This is one of the most simple steps in the process. The barrel needs to be glued into the end of the flashlight so that it is centered, and placed high enough so that there is room left for the piston, and about 3mm of piston travel. To get the highest velocity possible the barrel length must be optimized for the pressure you will be using, I ran through a variety of pressures through a simulation called GGDT
100psi- 8 inches- 600 Feet per second 200psi-12 inches (suggested, easy to pump to pressure, and almost supersonic velocities (850 feet per second) 300psi-24 inches- 1000 Feet per second
The barrel I recommend is a 4.5 mm internal diameter brass tube, because this will leave you with substantial air flow, and ammo can be bought as steel bb's from the store. A centering jig for the barrel can be achieved by wrapping a bamboo skewer in tape, so that it is centered with the main body of the gun.
The piston is one of the most tricky and important steps in the process. The piston must have a sealing face so that it seals against the barrel. It also should have enough room on the sides for air to get past and fill the space beside the barrel, however it also has to be big enough, so that when you trigger the piston, all the air doesn't vent out the back. The most efficient way to make a piston is to stamp it out of thick rubber, and then test it with the gun, until it works. You dont want to sand the piston too much, or you will have to remake it. The attached picture shows how it works.
* A piston can also be made from plastic, with a rubber sealing face, and then wrapped in tape till it fits*
Now we want to make a valve assembly that can be easily removed and put back in place, so that the piston can be accessed easily if needed. Take your tire Shrader Valve, and shave all the rubber of it, then sand it rough. Next take the battery cap of your flashlight, take the spring off, and drill a hole to fit the Shrader Valve, then insert the valve and epoxy it. To make it airtight, wrap plumbers tape around the threads.
After everything has dried for at least 24 hours, put the piston in, and thread the Valve end cap on. Pressurize the gun, THEN INSERT AMMO, and remove the pump, to fire push the pin in the valve. Estimated velocity, and simulated velocity were near 850 feet per second. Be Careful and Have Fun!
Step 5: Troubleshooting
Gun isnt shooting when valve is pushed? The piston is too tight, and it must be shaved down, remove it and use trial and error to make it perfect
Gun vents all air when valve is pushed? The piston is too loose, and it must be remade or wrapped in tape.
There are leaks but I cant find them? For a small launcher like this, leaks can be hard to find. To find them, pressurize the gun and put in in water, find the bubbles and epoxy it
Threads are leaking? The pumbing tape must be tight, and pliers may be needed to attach the valve cap.
Picture Credit: Google Images Spudfiles.com (Specifically Mr.Crowly and Jacksmirkingrevenge) Myself