Dealing with Contreras

 is a quest in Fallout: New Vegas.

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

Detailed walkthrough
After talking with Contreras and passing a speech check or paying some caps he will send you to find Isaac at Gun Runners. When/if you find Isaac (due to a bug he has been known to disappear)you must either pass a speech check, barter check or collect 100 pieces of junk metal for Isaac before you can return to Contreras.[ Approx. 100 Tin and Bent Tin cans can be found in Aerotech Suite 200, located within the Aerotech Office Park to the south-east of Gun Runners ] If you cannot seem to find Isaac then go to his house to the east of Gun Runners. Return to Contreras and he will reward you with caps and your next mission, he will send you to Blake in the Crimson Caravan (he is the merchant standing next to the display rack) and simply collect a shipment of "medical" supplies, you can point out to Blake with a high enough Science or Medicine that the supplies in the package could be used for Chems he just comment that the Crimson Caravan does not deal drugs. Return to Contreras again and you will be payed caps, then Contreras will tell you to wait a day or two. Return to Contreras after a day and he will direct you to Miguel's Pawn Shop with the package from before where you must deliver it to Price. When you arrive you will find he is not there, instead talk to Keller who after talking with him will reveal he is an undercover NCR Ranger, at this point you can kill him, bribe him (500 caps) or help him arrest Contreras. Return to Contreras with the news if you bribed the ranger or killed him, he will reward you with the unique rifle "This Machine" if you killed him otherwise you must go back and kill the ranger or help him deal with Contreras. Simply talk to Lt. Boyd after agreeing to help the ranger and the quest will end, yielding the same reward as helping Contreras, the rifle "This Machine", only Contreras will no longer be available as a merchant and will not be replaced (?) with another merchant.

Bugs
Sometimes your contact for the third errand will not show up.

The repsonces 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.