How was Andrew Payne, the gun expert hired for Hinton's trial discredited?

Andrew Payne was to show that the bullets from the murders and robbery did not match Hinton's mother's gun. Payne did not know how to operate the comparison microscope when he went to the laboratory of the Department of Forensic Sciences. That information was brought out during Hinton's trial, as well as the information that Payne had poor eyesight. So his testimony was discredited.