A hybrid gun for launching a projectile by a combined propulsion effect of a chemical propellant and electrothermal energy. The gun comprises a breech-associated electrothermal propulsion energy injector having a plasma beam generator and a tubular chamber holding a light working fluid that during the travel of the projectile in the barrel produces at the rear of the projectile a light gaseous buffer zone. Optionally the gun may have in addition one or more electrothermal propulsion energy injectors mounted on the barrel.There is also provided ammunition for use with a hybrid gun having a chemical propellant holding cartridge designed for ignition by working fluid injected by the breech associated electrothermal propulsion energy injector.