Abstract: As a result of the thin tube wall, the temperature of the tube rises readily during rapid firing and even reaches to a scorching hot condition (over 300°C). Thus，it will have influence on the ability of sustained fighting and this has become one of the critical problems in developing mortars with high efficiency, An emphasis is given to the discussion on the contradiction between the increase of effective coefficient of the metal tube and the requirements of firing efficiency, which is due to the temperature rising of the tube during rapid firing. Furthermore, a formula for calculating the rising temperature is derived and described and examples are given。