Never heard of the gun, so I looked it up to find out its caliber.

Turns out it's a 12-ga shotgun, and this guy shoots his.

Richardson Industries "Guerilla Gun" Slam Fire 12 gauge Shotgun @ YouTube

Interesting...

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These will do the job. Back when the govt. was creating new gun laws to save us, my father who was a master tool make and worked for a company which had access to a whole bunch of different sized seamless tubing made a bunch of these in different bore sizes, from .410 up to 12 guage. They were simple looked like a machine jack and carried in two pieces. The outer piece had a machine nut welded on it and a bolt screwed into it with striker point on it (One piece. would expand the ends so a shotgun shell would just slide in with a good fit. You slammed the to pieces together to fire it, pulled the barrel piece out of the striker tube, reversed it and inserted a fresh shell replace the barrel tube and this would expel the fired shell. This was sometime around 1967 or 68 as I remember. You could get a number of rounds off very fast and with good control. Light field loads were recommended but these did rattle the windows sometimes. He made a number from stainless steel tubing, as well as bronze tubing. Laying in a tool box or in the trunk of a car they looked harmless. He retired shortly after this and always carried several with him on his travels around the U.S. Never had to use one, I also enjoyed experimenting the things. We did make several that used high pressure valve and seamless tubing to fire 38 specials. Just something different. Don't know what happened to them over the years. He did give a few to fellow travelers. ATB