I originally thought that all the weapon parts were in specific grey metal crates, the ones that require a small QTE to open. However, I just received a handgun part from a normal salvage box. Are weapon parts all in specific crates that I have to look for, or are some of them received from opening a certain number of small salvage boxes?
Weapon parts are pesky. They can be found in any salvage boxes, small or otherwise, at a seemingly random rate.
In addition to being able to find them in salvage boxes, you can occasionally snag them from the chests at the ends of tombs, as well as looted off of dead enemies.
So, in order to maximize your chances of acquiring weapon parts, open every salvageable container you find, do every tomb, and kill and loot as many enemies as possible.