The gun, which Degorski and convicted murderer Juan Luna allegedly used in the 1993 murders in Palatine, was never found.
Luna, who was convicted of the murders back in 2007, was sentenced to life in prison.
Defense and prosecutors are expected to give their closing arguments Tuesday.
Seven people were herded into the restaurant's walk-in cooler and shot and killed.
Both men were arrested after Degorski's former girlfriend told police 10 years later that he confessed to her about his role in the crime.