A soft-recoil or FOOB (Fire-Out-Of-Battery) system can reduce the recoil force considerably. Its firing sequency is different from that of a conventional or FIB (Fire-In-Battery) system. In FOOB system, the gun is latched and preloaded in its battery position prior to firing. When unlatched, the gun is accelerated to the forward direction and then the forward momentum of the recoil part is generated. Since this momentum reduces the recoil impulse, the recoil force will decrease significantly. When designing the soft-recoil system it is important to design the forward momentum profile of a recoiling part. In the present study, the method to determine the forward momentum has been studied and its optimum value has been obtained theoretically. The numerical simulation of the soft-recoil system is performed to show that the present soft-recoil system works functionally well.