I have discovered a way to obtain said weapon, which according to Fallout Wiki is the second most powerful weapon in the game. I disagree with that, however, as I'd put "Euclid's C-Finder" at # 1 (given that you directed power to Archimedes II during the "That Lucky Old Sun" quest), then the "Alien Blaster" at # 2, "Pew Pew" at # 3, and then the weapon of topic for this thread, the "Mysterious Magnum," at # 4. In any case it's pretty darn powerful (and seems to have a low degradation).
You can obtain this weapon during the "Talent Pool" quest obtained from Tommy Torini at the Tops Casino (you will want to have at least 50 barter before this quest in order to obtain this special weapon). One of the talents you will need to recruit for this quest is "The Lonesome Drifter." He is located in front of the Sunset Sarsaparilla sign near the El Dorado Gas & Service. If you've started the quest, then he'll he'll have a map marker.
Basically at the end of your conversation with him, and as relayed earlier you have a barter of 50 or higher, you'll have an option to ask for compensation for finding him work. Do so, and he'll give you the "Mysterious Magnum." The other cool thing about this gun, besides for it's damage, is what happens when you draw it or holster it. When you draw the gun it plays the same little tune that's played when the Mysterious Stranger shows up in VATS to lend a hand, and another shorter audio clip when holstered. If you have a sneak of 50 or higher, then it can also be a very handy holdout weapon (not taken away at casino's and such).
Or, if you're a baddy, you could always just kill him
Is The Lonesome Drifter the son of the Mysterious Stranger?
Here's a link to a video on YouTube as well. It's made from the PC version, but the dialog choices are the same (also it's not my video):
I know that Fallout New Vegas is a large game with a TON of cool little finds, but this one seemed worth sharing. Have fun!
