After finishing up Toil and Trouble, go to the Arid Nexus - Badlands and kill Skags until one of them drops an ECHO Recorder. Picking up the ECHO will start this mission, which will task you with collecting four pieces of a gun that Marcus will put back together for you. Of course, these four pieces have been eaten by the very same Skags, so you’re going to need to go hunting.

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You can start the mission by checking the ECHO that randomly drops from a Skag (left). The gun parts also randomly drop Skags (right).

The four pieces of the gun are completely random and can drop from any actual Skag that you kill. The only problem with this is that it can take a long time to actually get all four parts. Common locations of a lot of Skags include both Arid Nexus areas, Three Horns - Valley, and Wildlife Exploitation Preserve. If you should run out of Skags to kill and haven’t gotten all four gun pieces, then simply save and exit the game to make the Skags respawn. Once you get all four pieces, return to the Bounty Board in Fyrestone (Arid Nexus - Badlands) to turn in the mission.