You should be able to unlock this by the end of Volga level.
If you pick up every extra part of any gun you find by holding O while being over the top of the weapon. You should be able to get this on either the Shotgun or Revolver. I had a part in each category for both after returning to Aurora with the Passenger Carridge.
A new barrell, gadget, Magazine, sight and stock are needed for the trophy.