You can have some dangerous fun with some pipe supplies from your local home store. This Steel Shotgun Pneumatic Cannon is a good example of that. For more items like this have a look here.
"The Steel Pneumatic shotgun is a very simple metal spud cannon, so see how to make it in this video. It is specially designed for C-cell batteries, but will work with any projectile (granted it makes a good seal in the barrel). It is also very easy to construct and disassemble considering it is put together completely by treaded fittings."
Via: Wonder How To
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Nipple == pipe with threading on both ends.
He is using a LOT of threading tape–and anybody who makes this should too, or else your gun will leak like crazy.
Unlike him, you should use two wrenches to tighten everything–not hand-tight as he is doing in the video. Tight == no leaks. Although the leaks would be slow (and not explosive) it is still annoying to have to deal with leaks. The only place where you don’t need tight is the barrel-side (which is the 30″ nipple) as the pressure dosen’t need to be contained but only for a second. And as also mentioned in the video, you can crack the plastic of the valve which is no good.