Goal of these missions is to collect 4 pieces of weapons, of which after the completion of the mission, a weapon of the appropriate type is obtained. This weapon is given as a reward for the mission.
"There are components to a shotgun scattered around. If you find all of these components, I can reassemble the weapon for you. Bring me the Body, Magazine, Stock, and Barrel."
Find all of the shotgun parts.
Hop in a runner and travel to the area marked on the map.
Magazine: It is located on top of the structure just behind the Catch-a-ride. Park at the station, go through the fence, on immediate right are boxes to jump on. Jump onto the brown structure.
Stock: It is on a box on the other side of the water. Go across the metal cat walk and the stock should be visible on a crate next to the building. Use that crate to jump onto the roof.
Barrel: It is on top of the structure which is next to the vending machines. From here face exactly NE and the last piece is visible on top of another structure.
Body: It is on a structure which is similar to the structure the magazine was on. From the top of the structure where the barrel was, make a running jump to the domed building on the right (while facing the water) and then do a running jump onto the platform. This can be complicated and may take a few tries but it is possible. It can be made easier by equipping an high explosive gun, approaching the run-up from the domed roof looking downwards, then firing the gun mid-jump. The jump can be made easier with Brick: the faster movement while Berserk translates to a further jump.
An easier approach is to get on the other side of the fence by going back across the water, down and across the water again and back up. Approach the end of the fence where it goes into the rocks and jump as if trying to get on top of the end of the fence. Landing point is half-way up, but from there it is possible to jump onto the top of the fence, then just walk across the fence to the structure with the shotgun body. This approach can activate a distant turret, but it can either be ignored or taken out.
An even easier approach is to get near to the building and jump until the Green Trophy sign can be seen. As soon as it shows up (usually during mid jump) simply press X (on 360) as it appears and grab it.
"Great! Here is your shotgun, good as new."
Some of the parts of the shotgun may spawn in a different order, but all locations are the same. Don't be surprised if you find the barrel where the stock should be.
The marker on the map is indicative of the general location for all pieces located near the marker. Their exact location is shown by the loot icon on the compass.
The items cards for parts has the following texts:
Magazine: It'll do in a pinch.
Stock: A previous owner's initials are carved into it.
Barrel: It looks like some insects were nesting inside.