This article has established a mathematical model to evaluate the influence of several structural parameters of the gas chamber on the movement of the gas-operated automatic mechanism during single and series firing. Parameters such as the area of the gas block hole, the gap area between the piston and the gas chamber, and the area of the gas exhaust hole significantly affect the movement of the automatic machine. The velocity of the bolt carrier, the movement cycle of the bolt carrier, and the pressure in the gas chamber are used to evaluate the influence of these parameters. Numerical calculations are applied to the PKMS machine gun. Calculation results show that: The maximum velocity of the bolt carrier corresponding to the number of gas exhaust holes on the PKMS machine gun is 0 holes, 1 hole, and 2 holes, respectively 4.913 m/s; 3.625m/s; 2.424 m/s. The calculation results are the scientific basis to evaluate the reliability of the gas exhaust section adjustment part during the firing process, providing the necessary energy to supply the automatic machine to work in adjustment cases. This result can be applied to survey and calculate the design of gas chambers for machine guns working on the principle of gas block.