Gun Run

Gun Run is a miscellaneous quest in Fallout 4.

Detailed walkthrough
As you approach the Rook family house in Salem, you will see Barney Rook standing on the roof of his house, which is surrounded by mirelurks. The mirelurks are initially friendly but will turn hostile when approaching the house, starting the quest. After they are defeated, get closer and Barney Rook will speak to you and invite you to enter the bunker in the basement of his house.

Upon arrival, Barney will speak to you, continuing the quest. He will tell you the Salem Militia, composed of himself and his rifle, Reba, need some help to reactivate the defenses of the town. He will ask you to activate the terminals controlling the turrets throughout the town. To do so, you must enter Barney's password in the terminals. As he warned you, you will find mirelurks along the way.

Once all five terminals have been activated, return to Barney and he will give you a reward: Barney's key, which allows access to Reba II, a special sniper rifle with +50% bonus damage against bugs and mirelurks.

Bugs

 * Barney may already be dead. His pre-death may play out in several ways.  You may approach Rook House and not get the first quest stage to trigger at all ("Kill 0/3 mirelurks"), despite the mirelurks being present.  Barney's corpse is on the roof, invisible, with Reba laying next to him.  Reloading a save, one might try to approach Rook House at a later time to get a different result: the quest activates and instructs you to kill the 3 mirelurks, but he's still dead on the roof, this time with his corpse visible.  This will result in "Talk to Barney" being an unfulfilled miscellaneous quest.  In both cases, his corpse was lootable and contained his basement key, so both Rebas are attainable even as the quest either gets stuck in your Pip-boy or never triggers at all.