Though the weapon has less damage than The Mauler, the Man Opener ignores normal damage resistance, similar to the Deathclaw Gauntlet. This property makes it more useful in some situations; for example, when going up against a Mirelurk Hunter or Enclave Hellfire Trooper. Perhaps the best way to describe it is to call it an "automatic Deathclaw gauntlet."
Enter the Supply Plant, then go east through the hallway. Turn north and continue until you reach two exits. Take the left. Continue through the passage, climb the stairs, and eventually exit into a large room. There's a bed on the left as you enter. Man Opener is on the bed.