This guide is going to show you how to get the Mysterious Magnum without killing The Lonesome Drifter or have 50 Barter Skill.
Location of the Mysterious Magnum
The Mysterious Magnum is a .44 revolver that plays the Mysterious Stranger theme when drawn or holstered. It´s owned by The Lonesome Drifter that can be found East of El Doraro Dry Lake.
Once there, you will approach the Lonesome Drifter when he is sitting on the tree trunk, you will open VATS or aim at the revolver to break it.
Once fired, he should have dequipped his weapon and you will finish the fight by holstering your weapon.
Now you will wait until he sits down again on the tree trunk, then you will go sneaking behind him, and you will pickpocket him, he will have the damaged revolver in his inventory.
Then you just have to repair the weapon with another revolver or with the Jury Rigging Perk, you can just use a 9mm pistol to repair it.
Well thats it, this is basically just to tell you, if you dont know it by now, how to get the weapon without the Barter Skill to have him for the quest Talent Pool.