The tip is a nail, the body is a cut-down tube from a Bic pen (with hole cut in black end cap), the two springs are from some spare pens, and the springs are threaded onto a second nail. The heads of the nails are touching, and the blue cap is the cut-down base from a blue wall anchor (like the kind used for nails) with the side tabs, base rim, and fluted tip removed.
What's there pretty much explains everything. What it is is a pen tube from a Bic pen with a hole drilled in the black end cap for the tip of a nail, then two nails placed with their heads touching were dropped into the tube so that the shaft of one nail protruded completely from the cap. Springs were threaded onto the second nail (the one not protruding from the cap), and a nail wall anchor was cut down and used as a cap for the back so that the shaft of the second nail could protrude from the wall anchor when the first nail was forced back into the pen tube by a shell.
I have since put the idea aside in hopes that a simple nail and spring put in between the layers of the bolt will be enough to eject the shells. If I can test this new "design," I will post the results and how effective of a system it is.