A few of the "mischevious" guys at work have made some of those but for smaller projectiles example: take a 6" long piece of welding filler metal and stick it into a ear plug, then stick that through a 1/2" tube that has a larger 3" tube with a valve welded inbetween them. Put 200psi of air in the 3" tube then open the valve on the 1/2" tube.
end result is a welding rod that will penetrate through a 5 gallon paint can
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