Dealing with Contreras

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
There are two ways you can complete this unmarked quest. The first, and much quicker way is to hack his computer, download the weapons manifest then report back to Lt. Boyd. The longer way is to speak with Contreras, he wants you to run several errands for him. Either route will get you a unique rifle This Machine.

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 and offer him the passphrase instructed in your last meeting with Contreras. Keller does not know the counter-phrase. As always in this game, there a number of paths you can take. You can just say good bye and report back to Contreras. You can just go with it and try to initiate a conversation with Keller regarding the package delivery from Contreras. Keller will then reveal that he is an undercover NCR Ranger and then attempt to arrest you. At this point you can kill him, bribe him (250 caps) or, with a Perception stat of 7, help him arrest Contreras. The option to give the evidence to Keller will only get you arrested and force you to attack Keller. If your stat is not high enough and you do not want to end Keller's life, then take the first option and say goodbye and report back to Contreras.

In any case, 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. On the other hand, if you bailed and returned, then Contreras will encourage you to return to Keller and murder him in order to stifle the NCR investigation. Before going out the door of the supply shack, go upstairs, hack the terminal (average), and download the manifest therein contained. You will need this evidence later.

Assuming you want to stay in the good graces of the NCR, return to Keller and follow this path: First challenge Keller with the fact that you know he does not work with Price. Unhappy that he has been exposed, Keller will then explain that he is an NCR Ranger investigating Contreras and that Price is now being questioned regarding his role in Contreras' scheme. Next, explain that you are just a bagman with instructions to kill Keller. This will raise Keller's spirits as this is just the evidence needed to arrest Contreras. You can attempt to bribe Keller here, express concern over Contreras' fate, or throw Contreras to the wolves. If you do the latter, Keller instructs you to return to Lt. Carrie Boyd and so bring the investigation to a close.

With a good sneak skill character and a silence weapon (sniper rifle suppressor works fine) you can kill Keller without any NCR fame drop.Stand in the left corner of the shop and take Keller out with a headshot,this solution give you the This Machine from Contreras and also upgrade his stash in his shop.When returning to Contreras you will get speak option"I convinced the Ranger to work with you.You're safe,but be careful" even you killed him.You will get This machine and still your NCR repututation will be same (Idolize) or same as you had before starting the quest.

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 have hacked the Supply Shack terminal and downloaded 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. If you hack contreras computer and then talk to Lt. Boyd contreras loses all of his merchant inventory although still functions as a merchant.

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.
 * 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.
 * Even with 7 perception the speech challenge which results in you helping Keller arrest Contreras will fail.
 * (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 after completing the third task and obtaining "This Machine" will still sell to you, but will not give you the goods you purchase, but will gladly take your caps.