Gun Run

Gun Run is a miscellaneous quest in Fallout 4.

Detailed walkthrough
As one approaches the Rook family house in Salem, they 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 the Sole Survivor and invite them to enter the bunker in the basement of his house.

Upon arrival, Barney will speak to the player character, continuing the quest. He will say that the Salem Militia, composed of himself and his rifle, Reba, need some help to reactivate the defenses of the town. He will ask the Sole Survivor to activate the terminals controlling the turrets throughout the town. To do so, one must enter Barney's Password in the terminals. As warned, one will find mirelurks along the way.

Terminal locations:
 * Two terminals are located on the opposite corners of the restaurant, just behind the church.
 * Another is in the church steeple.
 * The fourth is on the upper floor of the Rook Military Surplus store, near Barney's home.
 * The last is in the alley opposite the church.

Once all five terminals have been activated, return to Barney and he will give the player a character 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 premature death may play out in several ways:
 * One may approach Rook house and not get the first quest stage ("Kill x mirelurks") to trigger at all, despite one or more mirelurks being present. Barney's corpse is on the roof, invisible, with Reba laying next to him. Reloading an earlier save and approaching Rook house at a later time may yield a different result.
 * The quest activates and instructs the Sole Survivor to kill the mirelurks, but he is still dead on the roof, this time with his corpse visible. This will result in "Talk to Barney" being an unfulfilled miscellaneous quest. The quest can be removed from the miscellaneous quest menu by using the console command.
 * In both cases, his corpse is lootable and holds his basement key, so both Rebas are attainable even if the quest either gets stuck in the Pip-Boy or never triggers at all.
 * After killing the three mirelurks in the first phase, sometimes the hatch which Barney disappears into will not become an interactive object, thus preventing moving forward with the quest. Fast travel away and back fixes this.