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 will attack you. 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. In either case, he will ask you to reactivate several turrets located 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.

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.