Gun Run

Gun Run is a miscellaneous quest in Fallout 4.

Detailed walkthrough
As you approach Rook House in Salem, you can see a man standing on the roof of a house, which is surrounded by mirelurks. The lurks 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 compound. Go to the basement- Barney seems to have no more dialogue while on the roof.

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 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. Already activated turrets don't seem to be of any help against them.

Return to Barney and he will give you a reward: Reba II, a special sniper rifle with +50% bonus damage against bugs and mirelurks.

If when you find him but he is dead on the roof, get the basement key from his corpse, then use console to resurrect him, you will meet him in the basement. If you choose not to resurrect Barney when you find him dead, you can loot his body for the basement key and get Reba II (and Reba, his original sniper rifle). However, this will result in the unfinished quest getting stuck in your Pip-boy data.

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.