Max fake ammo boxes planted

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.

My cousin encountered this, and I believe what he did was port to Shantytown (since enemies respawn there most of the time) killed enemies and just looted them. Then just repeat the process until you get one from an enemy. If you want you can try your chances at the Hillside Bunker or Research Lab after trying this.

Be sure to use your rifle to kill them, it would be better if you have the Scavenging skill from the Survival tree to increase your chances.