The ballistics info on the computer shows an image of the rear of two 9mm casings. The description under the left one states: "Distressed Bullet - Removed from fatal gunshot wound of Payn, Oscar". The casing wouldn't have been removed from the wound, the bullet would have been.
If Al's gun was taken at the range to be used to murder Payn, it shouldn't have matched the weapon taken from him by IAB. There was never a chance to get the murder weapon back in his possession.