Originally posted by Docfeelgood
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Get two steel pipes, one of .75" interior diameter, the other 1". Put the nail through the end cap, screw it on to the larger pipe. Insert a shotgun shell into the smaller pipe; it'll headspace on its rim and remain in place at the end. Slide the smaller pipe into the larger one, and pump them together. The nail will strike the primer of the shell and set it off.
Making a gun like this is not illegal in the United States, but in order to give it to someone else you have to get a federal firearms license (FFL).
edit: I made a slight mistake here; you put the nail in the end cap with a 1" circle of cardboard. Nail goes through cardboard, cardboard gets shoved into the end cap, screw the cap on. Cardboard is held in place between the end cap and the pipe.
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