The effects of bias magnetic field on the discharge characteristics of a coaxial gun and the parameters of ejected plasma are studied. At constant initial conditions (charging voltage, gas-puffed), magnetic probe data at different axial positions reveal the influence of the external bias magnetic field on the current distribution within the annular flow channel. Both photodiode signals and plasma radiation images show the plasma ejection characteristics. It is found that either applying or not applying the bias magnetic field results in two types of plasma with distinct ejection characteristics: spheromak or jet. By comparing the plasma parameters with and without bias magnetic field, it is found that the variation of jets velocity and momentum with discharge conditions are consistent with the snowplow model, whereas the spheromak plasma is subjected to a retarding force during the ejection, which decelerates the spheromak. Irrespective of the bias magnetic field, an increase in the discharge current amplitude leads to an elevation in electron density and temperature. The density of the spheromak is lower than that of the jet, but the temperature is higher. This is due to the fact that the electromagnetic focusing mechanism in the jet leads to a further increase in density, while the magnetic field lines stretched during the formation of the spheromak inhibit the focusing effect and expand freely after ejection. In addition, the magnetic field stretching process consumes part of the axial kinetic energy from the plasma and converts it into magnetic energy. The subsequent magnetic reconnection event causes the magnetic energy to be dissipated resistively by heating the plasma and then be converted into thermal energy. These results give us a better understanding in the interaction between plasma and magnetic field.