In Tomb Raider (the 2013 edition) for the PC, PS3, and Xbox 360, one part of upgrading your gear requires you to collect Weapon Parts.
The big question has been, are Weapon Parts set to specific locations in the game, or are they random?
Weapon Part locations are indeed random, and can be found anywhere you would find Salvage. Boxes, crates, dead bodies, dead animals, etc. Weapon Parts can literally be found anywhere, so always make sure you’re searching every nook and cranny in a map while your’e exploring.
Once you have all of the Weapon Parts you need for the current upgrade, just visit the base camp and sit at the camp fire and your weapon will automatically be upgraded to the next version.