An existing projectile (21) is modified by adding a tail piece (22) to enable axial stacking of multiple projectiles (20) in a common barrel. Propellant for each modified projectile (20) is contained in respective chambers located external to the barrel and connected to the bore of the barrel through associated ports. Tail piece (22) aligns with a respective port and provides a space between consecutive projectiles (20) into which the propellant gas expands after ignition. Separate claims are directed to tail assembly (22), modified projectile (20), the barrel assembly having a plurality of projectiles (20) stacked in end-to-end relation, and to an external initiation system for the barrel assembly (see FIGS. 4, 7).