Aiming at the phenomenon that the gun stop firing by the separation of the broken primer and the cartridge casing during the cartridge ejection process of high-speed automatic gun, the trajectory of broken primer is analyzed. Through the working principle of actual gun cartridge ejection process and the application of virtual prototype technology, the dynamic equation of cartridge ejection process is analyzed, the cartridge ejecting mechanism is established, the parameters indexes of contact time were calculated and the cartridge ejection process was simulated by virtual prototype. The results show that the separation of the broken primer and the receiver, the hook, the flap may cause the gun to stop shooting when the breechblock velocity is greater than 17.6 m/s.