Hi ajsgirl,
I haven't played this game, but this is from the walkthrough:
Lab - Assembly Table, add adapter from the CSI Reserves. Take gun apart and put the adapter on it. Comparison Microscope, compare the bullet from the gun club to the bullet from the body. Then compare bullet from the .45 automatic gun with the CSI Reserves adapter and bullet from the body. Turn one of them until they line up, confirm match.
Hope that helps, let us know if you still have problems.
