The concept of stacked projectiles (multiple projectiles loaded nose to tail in a single gun barrel with propellant packed between them) predates Metal Storm. Roman Candles, a traditional firework design, employ the same concept. However, in the case of Roman Candles the propellant which propels the leading projectile from the barrel continues to burn in the barrel, igniting the charges behind each subsequent projectile in turn, ensuring that all projectiles in the barrel are discharged sequentially (and inevitably) following from the single ignition. Various methods of separately firing each propellant package behind stacked projectiles have been proposed which would allow a 'single shot' capability more suitable to firearms.
Mike O'Dwyer, an Australian inventor, observed that these methods did not eliminate the problem of unintended propellant ignition caused by hot gases 'leaking' back up the barrel. Mike O'Dwyer's original Metal Storm patents demonstrated a method whereby projectiles placed in series along the length of a barrel could be fired sequentially and selectively without the danger associated with unintended propellant ignition.
In the original Metal Storm patents the propellant immediately behind the projectile closest to the muzzle of the gun barrel was ignited by an electronically fired primer, the projectile was set in motion, and at the same time a reactive force acted on the remaining stacked projectiles in the barrel, pushing them backwards. By design, the remaining projectiles would distort under this load, expanding radially and sealing against the gun barrel wall. This created a seal which prevented the hot propellant gases (expanding behind the lead projectile) prematurely igniting the remaining propellant charges in the barrel (blow-back). As each of these propellant charges was selectively (electronically) ignited, the force 'unlocked' the projectile in front and propelled it down the gun barrel, and reinforced the radial expansion (and hence the seal) between the projectiles remaining in the barrel and the barrel wall.
Subsequent designs discarded the 'distorting shell sealing against the barrel' concept in favor of containing the propellant in 'skirts' that form the rear part of each projectile. These skirted projectiles differ from conventional 'shells and cartridge' units in that the skirts are part of the projectile, and in that the skirts are 'open-ended' (at the rear). The rearward seal to the skirt is provided by the nose of the following projectile in the barrel. As in the previous design, the firing of a projectile results in a rearward impulse on the remaining projectiles stacked in the barrel. This results in the skirts of the remaining shells in the barrel being compressed against the following shell heads, effectively creating a seal that prevents hot gases in the barrel triggering unintended propellant ignition ('blow-back') along the length of the barrel. Metal Storm also introduced inductive electronic ignition of the propellant, effectively from outside the barrel. This overcame technical issues in maintaining physical contacts with the propellant charges, which due to the compression effectively 'shift' slightly backwards within the barrel during firing.
The skirt-to-nose joint has in recent designs incorporated an easy-release arrangement which allow the shells to be clipped together to form robust ammunition 'tubes' which can be transported more readily than individual shells, and inserted directly into Metal Storm barrels. Metal Storm have indicated the tubes can be 'pulled apart' and reconstructed in the field to make up custom combinations of ammunition, and to facilitate 'topping up' a partly discharged tube that is still in the barrel.