it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing the gun was empty.

This is a reference to the mad dog incident. The feeling inside the courtroom was somewhat like when Atticus shot the innocent mad dog as it came through Maycomb; Atticus ended that poor dogs life and the jury was about to do the same to Tom Robinsons by sending him to jail.