Abstract: A recoilless device using double nozzle was presented to reduce the loss of bullets' velocity and improve the recoil efficiency. Based on kinetic theory of supersonic gases, considering the after-effect process, a gas dynamic model of large-caliber weapons was established for the interior ballistic. The mathematical model of gas dynamics was proved to be effective by experimental tests. The properties of system parameters of two recoilless devices including muzzle velocity, anti-recoil efficiency, motion characteristics of automation and recoil force were compared and discussed. The results show that reducing recoil efficiency of large-caliber weapons using the recoilless device with double chamber is remarkably improved and recoil force is reduced effectively.