look at their paintball gun designhttp://www.rotteneggs.com/r3/show/se/189689.html
they use hot glue to glue a piece of pipe to the barrel with a tee connector.
they dont glue the barrel
they havnt heard of end caps and just use electricians tape
they make a hopper out of a "spudgun barrel" and lids for a can.
they insert a fire extinguisher into some pvc pipe but drilling a hole and inserting the nozzle of it into the hole.
anyway seems like a good idea i might build one.....but safer....and more powerful
