Recently there has been a lot of interest in bb machine guns. I decided to join in the fun and made one of my own. The bb machine gun works on a few neat principals. Thanks goes to spudsonfire for posting his vortex design! The bb gun is fed air with an air compressor. At the bottom of the vortex tee, you will see a blowgun setup. This standard blowgun acts as a triggering system for the gun. Air blows up from the blowgun when triggered and into the vortex disk, in the meantime the tee is pressurizing. The combined efforts of the pressurized chamber and the vortex disk cause the bbs to shoot out at a very fast rate of fire. It seems like a very complex concept, but it really is a simple mechanism. The bb's just get stirred around in the vortex and head out the barrel. There is a high flow air stream in the barrel and the bb's just follow the air out of the barrel. If you have trouble seeing the concept, just try to see it in a simpler way, it seems much more difficult than it actually is.
There are also "side plates" on the vortex disk. These are there to prevent jams due to the weight of the bbs. I believe that the side plates also help vortex action. Here are a few pics of the launcher:
The vortex disk was cut from 1 5/8" solid PVC rod from mcmaster.com. With a little grinding on the inside of the 1.5" female adapter, the rod is a perfect diameter for a 1.5" vortex tee. Dimensions are not crucial, honestly you can just eyeball the thing. The vortex hole is about 3/4". The other important part is the barrel. It can be made with either aluminum or stainless steel tubing from mcmaster. The ID is .18" and the OD is .25". I highly recommend the stainless steel, it costs a bit more, but the aluminum stuff is pretty horrible. It works, but it is very easy to bend. Once it's bent it is impossible to straighten. You can sleeve the aluminum into another size PVC. The stainless is so much stronger it wouldn't really even need sleeved. Here are some pics of the procedure:
I performed a slight vortex mod. The metal plate was part of the original design, just I hadn't taken pictures of it. The cream colored stuff is Durham's Rock Hard Water Putty. JBWeld or Bondo could be used also. The purpose of this mod is to create a sort of "funnel" for the bbs to get into the vortex chamber. Before the mod, the last 20 or so bbs were very difficult to get into the vortex disk, you would have to shake the gun for them to fire. Now with the mod, it fires every last bb with no shaking at the end. There is a bit of shaking needed once during the firing process, and I am working on that now. Soon I should have all stalls in shooting taken care of.
Attaching the barrel is another aspect to consider. You can do it several ways. The easiest ways are as follows: Method 1(top) is to use 1/4" sch80 pipe over the barrel. You can thread the end of it, and tap the tee. The barrel stays in the sleeve by friction. Method 2 (bottom) would be to use a compression fitting, this is the easiest method, but would only work well with stainless barrels.
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