This is the FIRST unique weapon you come across in the game and is pretty sweet. At the end of Vault 111 when standing in front of the overseers desk to the right of you is a small fenced in room, on the wall in that room in a case that is locked by a master lock. Pick the lock and its yours!

Constant fire freezes enemies making them unable to move

Vault is 111 is in the top left hand corner of the map, I dont think I need to include a photo.

Here is a video on where to find the gun AFTER leaving the vault.