Virgil's rifle

Obtained by Crafting it, stealing it from Virgil's laboratory or convincing him to kill himself by lying about the serum.

Convincing Virgil to kill himself
After gaining entry into the institute, head over to the Bioscience section as soon as you can, go through the series of doors and into the FEV lab, inside, kill the few turrets there are and once you reach the room with the super mutants in glass cylinders in it, go to one of the containers there and grab the serum. After obtaining the serum, go back to Virgil and if your charisma is high enough, convince him that the serum isn't there and that his condition will just get worst and he'll have to kill himself to end it. He then tells you to do it for him, after you do it, pick up the weapon from his corpse.

Stealing it
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