Dealing with Contreras

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
Contreras wants you to run certain errands for him. Doing so will get you a unique rifle This Machine. You can also turn him in to Lt. Boyd and receive the Rifle.

Detailed walkthrough
Contreras can be found in the Supply Shack at the back of the terminal building at Camp McCarran. Enter the terminal, go up the escalators, down the hallway to the concourse, bear left to a down ramp, and bear right to the door.

You need to be at least level 12 before Contreras give you the dialogue option. After talking with Contreras and passing a speech check or paying some caps, he will send you to find Isaac at Gun Runners.

Isaac is normally at the work bench at Gun Runners. You must either pass a speech check, or a barter check or collect 100 pieces of junk metal for Isaac before you can return to Contreras. [More than enough Tin Cans and Bent Tin Cans can be found in Aerotech Suite 200, located within the Aerotech Office Park which is southeast of Gun Runners and due east of Camp McCarran.]

If Isaac does not appear at the work bench, go to his house to the east of Gun Runners. He will be in his house or will usually appear as you exit the house, and will run to the Gun Runners kiosk location.

Return to Contreras and he will reward you with caps and your next mission, which is to pick up some "medical supplies" from Blake at Crimson Caravan. Return to Contreras who will pay you can tell you to wait a day or two for the next mission. You can just Wait for 24 hours and then for another 18 or so hours and then try him again.

He will direct you to deliver the "medical supplies" to Price at Miguel's Pawn Shop. When you arrive you will find he is not there, instead talk to Keller who will reveal that he is an undercover NCR Ranger. At this point you can kill him, bribe him (500 caps) or help him arrest Contreras.

If you bribe or kill him, return to Contreras with the news. If you killed Keller, Contreras will reward you with the unique rifle "This Machine"; otherwise, you must go back and kill the ranger or you can change your mind and help him "deal with" Contreras.

If you choose to help Keller, talk to Lt. Boyd and the quest will end, which will get you the same rifle, "This Machine". Contreras will be moved to the Prison Cell Block, directly opposite from the room that Silus (Legion prisoner) is kept. Contreras can still be used as a merchant but his inventory doesn't get replenished.

Bugs

 * Sometimes your contact for the third errand will not show up.
 * The responses for the third errand are all messed up if you choose to kill the officer first.
 * After the third errand he will disappear and won't be where you previously talked to him.
 * When you first speak to Contreras there will be no option to start the quest, just to browse his inventory and to leave.
 * Isaac may disappear from the game.
 * After delivering the medical supplies/chems, when Contreras tells you to 'come back in a few days', you are given the dialogue option to say "So where was I supposed to meet Price, again?" even though you haven't been given that part of the quest yet.