Traditionally, on an over under, you put the more open choke on the bottom (fired first) and tighter choke on top (fired second). I'm not sure if the Norinco has the feature, or not, but typically, you can select which barrel you would like to fire first by shifting the safety left/ right before disengaging. (There may be times when you want to fire the tighter choke first, for example, on a long shot where the more open choke may be inappropriate, or if firing singles (turkey shoots, trap, etc...)