For improving shooting speed of large caliber cased telescoped ammunition (CTA) machine gun, a kind of gas-operated automaton was designed. Taking certain type 12.7 mm CTA machine gun as example, the opening and locking action was realized by a lifting and lowering chamber, the structure of the automaton was designed, and the virtual prototype model of the CTA machine gun was established by using ADAMS, the dynamic simulation analysis was carried out, and the dynamic characteristic parameters of the automaton were obtained. The simulation results proved the feasibility of the automaton structure, and provided a reference scheme for the design of large caliber CTA machine gun.