Abstract: The MECCAD code has been developed within Giat Industries to predict the heating and the cooling of a gun barrel during a burst. When the projectile moves in the barrel we calculate the convective parietal heat flux by using the Reynolds-Colburn analogy. After the projectile exit, the discharge of the propellant gas in the barrel is modelled with enough accuracy to respect the physics of the phenomena. The main difficulty is to predict the inner flow when the muzzle conditions are subsonic. The results allowed determination of the parameters of the expansion laws, which were used as the inputs of the cooling convective and radiative heat exchange laws. Thus, it is possible to calculate the initial barrel temperature just before a new firing. 120-mm gun calculations with MECCAD code were carried out for one shot with Armour Piercing Fin Stabilized Discarding Sabot (APFS-DS). The comparison with parietal temperature measurements shows a good agreement (especially near the forcing cone).