Abstract: A liquid propellant modular gun has a slim profile and is constructed for wide latitude in gun cluster configuration.The modular gun has a stationary barrel and is externally driven and cam operated by a drive cam and a control cam.The drive cam has one internal spiral cam track for driving the bolt forward to a projectile firing position and another internal spiral cam track for driving the bolt rearward to a projectile loading position.The control cam is mounted for rotation at the forward end of the drive cam and controls the injection of liquid propellant into the combustion chamber and an electrical igniter.A water injection mechanism is also associated with the control cam for injecting a small amount of water into the combustion chamber after the firing of each round to cool the combustion chamber structure by internal water cooling. The water injection mechanism is also effective to purge propellant from the combustion chamber in the event of a misfire.