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 will give you the dialogue option. After talking with Contreras and passing a speech check of 50 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 of 80, or a barter check of 80 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. This is poorly worded. You never get an option to help Keller with evidence, he takes the medical supplies and wants to charge you with a crime as well. You can pass a Speech check 60 to convince him of your innocence.

If you choose to help Keller, talk to Lt. Boyd and the quest will end, which will get you the same rifle, "This Machine". There is no option to talk to Lt. Boyd about Contreras unless you hack the Supply Shack terminal and get the manifests that Contreras has on it, proving his guilt. Only then will you get "This Machine". You cannot "choose" to help Keller, and he has no control over the quest reward. 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.
 * Isaac may also be killed by Fiends while going to and from his home and the Gun Runner's kiosk.
 * 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.
 * (XBOX360) The dialog when you give Contreras up freezes the game.
 * (XBOX360) After the quest is completed and Contreras is locked up in one of the cells, it is possible for you to initiate dialog with him (via 3rd-person view) and browse his wares. However, the game will freeze upon exiting the buy/sell screen.
 * (XBOX360) Contreras third task - If you go the path to report to Contreras that Price was not there and the person didn't know the password and using the 70 barter check with Keller to work a "deal" with him and Contreras you can get locked out of getting the "This Machine" gun from Contreras because even though you work out the deal with Keller to spare Contreras you are told to go back to Contreras and let him know. The dialogue option to say you worked things out for him is not there. (FIX:) However, if you choose to kill Keller (preferably by stealth boy and reverse pickpocketing a frag mine) you will not get negative fame and the dialogue option to get the gun is available in most cases.  Also talking to Boyd does nothing there is no dialogue since you worked the deal via 70 barter check with Keller.