Picture of Make a fully automatic BB gun for under $5
I don't know if this has been seen on Instructables , but here's my version of a fully auto bb gun.  This bad boy shoots about 15- 20 bbs a second.  Its really cool and destroys cans and cardboard. Please be careful, this gun will also destroy glass, windows, clear coat paint jobs and human skin!!   By the way.... all these items i had lying around. Total cost: $4.95---the cost of the bb's
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Step 1: Items needed....

Picture of Items needed....
Here's what you need:
1- Broken spray gun or air nozzle
1- Male hose fitting
2- Hose clamps
1-3/4" x 3" double-end threaded pvc
1- 3/4" pvc cap
1- 3/4 " I.D.(inside diameter) x 6" pvc or equal Tubing
1-1/4" x 24" metal tubing
1- 1/4" I.D. (inside diameter) x4" threaded tubing
2- Nuts
Epoxy or J.B. Weld
3/8" drill bit and drill

Ready.... here we go..

Step 2: Assemble....

Picture of Assemble....
Assemble the barrel first to give the epoxy time to set up. Start by threading your two nuts about 3/8" up the tube. Now insert the long smooth tube into the threaded tube and add a bit of epoxy to hold it tight.

Step 3: Drill....

Picture of Drill....
Drill a 3/8" hole about in the middle of the small threaded pvc. This is where your barrel will attach. Now secure this piece of pvc to the chamber tube using a hose clamp. I used part of my moto gas tank tube. It was the only thing i could find in my garage, but proved to be  strong enough.  Now attach the other end to front of spray gun or air nozzle with the other hose clamp.

Step 4: Screw...

Picture of Screw...
Screw the barrel to the chamber using teflon tape or thread lock to prevent a blow out or air leaks. Then screw on the cap, and attach the hose fitting on the sprayer or nozzle.

Step 5: Fire.....

Picture of Fire.....
Fire up your compressor to about 50 or 60 psi and check for leaks. Also check trigger mechanism. If alls well, crank up your air to 120 psi. I dont recommend a higher psi level for this gun.  Now fill up the chamber...and   fire away!!!
 Is there any way to do it with a Co2 tank from something like a paintball gun?