Put the cocking knob back on and pull the cocking knob back till it stops and rotate it to the safe position. Remove the screw on the left side of the reciever (between the breech opening and the rear of reciever. The whole insert will be free to remove, just pull to the rear. Since the rifle hasn't been cleaned it may take some effort to remove the hammer assembly but it will come out the back of the revciever then you will be able to insert a socket with extension and remove the breech plug.