Experimental MIRV (Fallout)
When it comes to the Fallout universe, most would point to the Fatman as the most fantastical weapon out there. Which is fair, until one learns about the Experimental MIRV.
In addition to launching miniature nuclear bombs, the MIRV comes with the added “bonus” of splitting said warheads into multiple smaller projectiles. Each one then detonates on impact, leading to a spread of nuclear explosions all around one’s intended target.
This is all well and good in the fantasy world of Fallout, but in reality, this would make using the weapon suicidally stupid to use. If the user was lucky enough to survive being so close to the blast radius of one shot from the weapon, they’d still have a higher than likely chance of succumbing to radiation poisoning from the nearby nuclear detonations.
As a result, it’d be impossible to see any situation where using this weapon would be advisable for a normal human being. They’d be better off using standard weaponry, or at the very least sticking to the Fatman and its single-projectile firing style.