Each UAV wreckage that player gets his hands on can be dismantled, using a multitool, yielding several random pieces from among those parts. Usually, two-three drones need to be disassembled in order to acquire a full set of the necessary parts.
There are two exceptions for that. The rifle stock needs to be acquired from a standard-type rifle or shotgun. The Gauss gun battery is made from a standard laptop battery, by combining it with some assorted small parts (with the help of a multitool and the Electrician ability).
Once the player has all the necessary components, he can proceed to put the weapon together. This process will require knowledge of both the Electrician and the Mechanic abilities, as well as a multitool.
Keep in mind though, you need 3 capacitors, and the stock is actually the rifle itself; scope and strap needs to be added later.