Abstract: A liquid propellant velocity assistance mechanism for a gun in one embodiment includes separate gas operated injectors for metering liquid fuel and liquid oxidizer into the bore of the gun after it has been fired and before the projectile has left the barrel. Each injector has an inlet connecting the bore of the gun to a gas receiving cavity section. An outlet extends from the liquid receiving cavity section to the bore. A piston separates the two cavity sections and is slidably moveable to increase and decrease the volume of each section. A valve normally closes the outlet between the liquid receiving cavity and the bore and is openable in response to liquid pressure. The high pressure gas behind the projectile when the gun is fired flows into the gas receiving section and pushes against the piston to create such higher liquid pressure.