From the pictures the shot gun appears to be more of a "zip gun" - a home made/improvised gun. The barrel is a tube while the shells seem to have some kind of squib or electric match replacing the shells percussion cap. That removes the weight of a mechanical gun mechanism with firing pin. This feels like a case for moving towards electronic firing case-less ammo perhaps?

Apparently it's one of these https://en.wikipedia.org/wiki/Davis_gun with two cartridges firing in opposite directions.