The purpose of a gun bore evacuator on the turret-mounted gun barrel is to prevent the entry of the gaseous products of gun propellant combustion into the turret compartment of the vehicle after the breech block opening. The mathematical model which takes into account all of the main parameters influencing the pressure change inside the cylinder of the evacuator was developed. Differential equations that describe the flow field through the nozzle in the barrel wall and in the cylinder of the evacuator were solved numerically. Furthermore, the numerical simulations were performed using the ANSYS FLUENT in order to analyze the phenomena occuring in the evacuator during the charge and discharge cycles which are difficult to be taken into account with the analytical model. Aforementioned simulations were performed with the 3-D model of the evacuator and with the initial and boundary conditions obtained from the interior ballistic calculations. In order to find the change of the static pressure in the cylinder of the gun bore evacuator, the experimental research was performed. The comprehensive comparison between the numerical results and the experimental data has shown good agreement. Based upon this analysis, the influence of the evacuator’s main design parameters on the pressure distribution and the temperatures in the critical zones was determined. The influence of the propellant’s initial temperature on the pressure and wall temperatures in the evacuator was also analyzed, due to the wide range of gun operating temperatures and the possibility of erosion occurrence.