Interior ballistics deals with actions within a barrel when it is fired. Often interior ballistics is used to answer question about maximum pressure in the barrel, muzzle velocity or muzzle pressure. To answer such questions knowledge is required of the propellant (the actual substance), its contents and how it burns; the shape of the barrel, its rifling and its chambrage and chamber volume; projectile weight and more. Here work is reported on development of computational tools for the simulation of the interior ballistics of non-leaking guns in several levels of complexity. These range from thermodynamic modelling to the detailed study of combustion of individual propellant grains.