Weapon Smith is an achievement/trophy in Metro: Last Light awarded to the player for fully customizing a weapon. This can be done relatively easily as early as Venice. The game seems a big ambiguous on what attachments you need to add, and how many - generally add one scope and all of the other extra attachments you can. For some reason though, the weapon does not need to be fully customised. For example, the Duplet does not need the Quad Barrel attachment, and no weapons need a laser sight. The best way is to collect the Lolife from Facility (the one needed for the A Present achievement/trophy) and add on any extra attachments you can.