All metal ofcourse!!
I bought a halon fire extinguisher at a flee market.
When I pushed that lever and saw the enormous cloud of gas coming out , I knew.
This thing could propell a marble machine gun.
A normal cloud or even vortex chamber would not work very well in my opinion because of the large dead volume.
In a cloud chamber the gas has to "punch" it's way through too many marbles to swirl them up.
This is no problem when u use bb's, but marbles are a bit heavier.
Anyway, it could be made easier for the stream of gas to lift those heavy marbles to the barrel port.
The idea is to get only one or two marbles at a time into the gas stream.
The diagram should explain my plan.
The shoulder stock holds the marbles, all neatly in a row,
waiting to enter the "cloud chamber".
The endcap of the mag, so to speak, is simply a ballvalve.
This would be best to reload quickly and seal properly.
The cloud chamber is 28 mm pipe, so the marble has some room to move around .The magazine is 22mm.
The mag is bent ( wich you can do with steel pipe)with one end pointing down towards the cloud chamber, the other end is,9 with the help of two elbows to get around the barrel,) connected to the ballvalve
The barrel should be .50" ( 1.25 cm?)
Maybe I'll add a rubber washer at the barrel entry to make the marble build up some pressure.
If I can finish this one in time I'll enter it in the competition.So don't copy this design yet.
I might add a CO2 bottle with regulator to keep the fire extinguisher up to pressure.Other thought would be to add a bottle to the ballvalve , filled with marbles to fill the reservoir very fast.
)and ask your parents for the bottle as something to hang in your room 
