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An electromagnetic railgun (EMG) fed by a capacitor bank was developed to launch hypervelocity projectiles. The efficiency of the EMG will ultimately determine the scale of its use since it determines the size of the power source. In practice, it is difficult to calculate the energy loss in the EMG system in a simple way, because of complicated physical phenomena such as dynamic friction occurring during firing. In this paper, an expression of the friction in the EMG system is conceived and validated by the measured current and in-bore velocity of the armature. With the conceived expression, a full circuit model, including the effects of the friction in the EMG system, is derived and validated. The simulated current and velocity based on the model are in good agreement with the measurements. The efficiency and energy loss in our EMG system are calculated and analyzed with the full circuit model. The primary mechanisms of the energy loss attribute to the impedance of the rail and pulse formation network.