An electrothermal gun uses an apparatus for generating high gas pressure. The apparatus includes a receiver having a combustion chamber for holding a propellant which produces a gas component and a particle component when the propellant undergoes an exothermic chemical reaction, and a flow passageway positioned downstream of the combustion chamber. An ignition mechanism causes the propellant contained in the combustion chamber to undergo the exothermic chemical reaction. A separator in the flow passageway substantially separates the particle component from the gas component in the flow passageway. The gun includes a barrel connected to the receiver and communicating with the flow passageway. By substantially stopping the particle component, namely metal oxide, from reaching the barrel, wear on the barrel is reduced.