- Fully Automatic BB Gun -
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I made two of these actually, one while I was in high school and the second a few years later. Both were very similar. They used an old BB gun barrel, some scrap metal and plastic pieces for the feed mechanism, a used CO2 cartridge for the BB reservoir and a small fire extinguisher as the compressed air cylinder, stock, handgrip and trigger. Both automatic feed mechanisms seemed to operate best between 50 and 100 psi. The fire extinguishers and gauges could be safely pressurized to 100 psi, which provided enough air to operate for about 5 seconds. I did test the gun once (very carefully) at 200 psi; it would actually shoot a BB through a steel garbage can. |
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 (Hold cursor over this text to view automatic operation.) |
The image to the left shows a cut-away section of the gun. BB's from the CO2 cartridge reservoir are prevented from dropping into the barrel by a pair of small magnets (shown by an "M" in the picture) and by the small end of the piston beneath. In the animated image, the reciprocating piston automatically feeds individual BB's from the reservoir into the barrel. When the fire extinguisher "trigger" is depressed, air enters the piston and cylinder assembly and blows the piston rearward. The pressure also travels up and through a gas tube to the space between the bottom two BB's, blowing the bottom BB downward from between the magnets into the barrel. As the piston continues backward, the air blast is then directed fully behind the BB and out the barrel. The air in the cylinder depressurizes, and the spring returns the piston back to its forward position to repeat the cycle. The animated image is in slow motion; the BB gun actually fires about 5 BB's per second. |
To the right you can see the disassembled parts of the gun. (You can click on the picture to view it in full 1024 x 768 resolution.) I drilled out the threaded end of the CO2 cartridge for the BB reservoir. The barrel is from an old BB rifle. I made the machine gun body from a 1" wide by 1.5" high by 2.5" long plexiglass block (actually, I cemented two 1/2" plastic sheets together to get the 1" wide block.) Inside the block you can see the 1/2" dia brass cylinder tube and the longer 1/8" brass air tube above it, which can be purchased from any hobby store. The piston beneath was made from an old screw, a few nuts and a plastic washer. The spring is a spiral-bound notebook spring. The end plate is a piece of 1/4" thick plexiglass. I drilled the 1" block for the barrel, brass tubes, air ports, magnets, etc. and threaded it carefully for the screws, CO2 cartridge, and on the bottom for connection to the fire extinguisher. I cut the brass schraeder valve stem out of an old innertube and threaded it into the fire extinguisher nozzle. You repressurize the fire extinguisher with the tire hose at your local service station, screw the machine gun body onto the fire extinguisher nozzle (remove the inner stem from the schraeder valve first) and the automatic BB gun is ready to go! I have had a lot of requests for plans or better directions for building this Auto BB gun. Sorry, but since I made these quite a while ago by hand without any original plans or drawings, I do not have anything better than this web page available. However, the gif image above is drawn to actual size (72 pix/in) and scale, and should give you a good idea of how the gun should be built. Here are a few more pictures that might help:
Hope this helps. Shoot safe and have fun.
Pete |
