Outside Boulder City revealed through the "They Went That Away" mission, exit the city and follow the road out of town and head west (left) when you come to the fork in the road. To the left you'll see a huge billboard with a man in a bounty hunter duster with a guitar next to him. This is the same guy you'll need to get a Mysterious Magnum from when you get him a gig playing at the Tops Casino in New Vegas. The entertainer and manager of the Pack Rats of Aces Theater will ask you to look for new talent for the theater, this is an official mission (can't remember the name) and the wayward point will point the way to him as well. Or, you can kill him and you get it off his body. Everytime you draw and withdraw, the magnum plays a small tune that sounds like the guitar.