The Prentor Expander (England 1912) This early Hydrogen Electric gun is believed to have been developed for the Kelly gang in England 1912, its actual origins are still a bit fuzzy. The police theorize that it was still under construction when the raid occurred, and the weapon was confiscated. The gun is extremely advanced in design The Prentor Expander could generate its own hydrogen from compressed water and an electric impulse. A small electric charge sparked from a hot coil would ignite the hydrogen gas and fire the weapon. This gun is believed to have been able to fire several types of projectiles; several darts, and several lead shaped projectiles were found when the weapon was confiscated by the Liverpool police department.