ok where do i get the quest that you end up getting this machine from? i know it has to deal with contreas or whatever his name is in mccarren

Head to camp McCarran, go into the Concourse. take left and go down the corridor...you'll enter the small shop with Sgt.Contreras. Talk to him and do a Free Quest: Dealing with Contreras. Complete it correctly and you will be rewarded with "This Gun"

"Correct" way