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. If you cannot seem to find Isaac then 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.

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.