It was called the MIRV. An experimental nuclear catapult. What made it different from all the other fat men out there in the Capital Wasteland was that it shot not 1, not 2, not even 3, but 8 mini nukes. At the same time. There was literally no reason to use this weapon, ever. It could kill pretty much anything if you had a halfway-decent big guns skill (and aim). It could also very easily kill you in the process.