Howitzer Gun is a type of large-caliber artillery gun that typically utilizes a mixture of propellant grain shapes to control the burning inside the gun, and to impart the prescribed velocity to the projectile with the smallest mass of charge without exceeding the maximum allowable pressure. In this paper, a mathematical model has been developed to predict the main internal ballistic parameters i.e., Maximum Pressure (Max. P) and Muzzle Velocity (MV) for artillery howitzer guns. The propellant charge utilized is a single-base propellant using a mixture of dual-shape grain namely, tubular and multi-tubular seven-hole shapes. The mathematical model was solved numerically using the Runge-Kutta method in MATLAB environment for a 130 mm howitzer field gun which was chosen as a test case. The simulation results were obtained in terms of several plots showing the effect of grain shape and grain mixture ratio on the Max. P and MV along the gun barrel. Furthermore, the optimum mixture ratio gives a slightly reduced MV with a significant reduction in Max. P has been determined graphically and compared with experimental data taken from the test firing of the gun. The simulation results showed good agreement with the experimental ones (less than 10% numerical error). The proposed model can be used in the analysis and optimization of other similar gun systems taking into account the right input data for both guns, and propellants.