The complete round weighed . It was long with a diameter of at its widest point; a subcaliber piston at the back of the shell was inserted into the launcher's barrel for firing. The M-388 atomic projectile was mounted on the barrel-inserted spigot via bayonet slots. Once the propellant was discharged the spigot became the launching piston for the M-388 atomic projectile: this was necessary because the fission round could not be subject to high acceleration and the spigot/piston, acting as a "pusher tube", facilitated this.