Fat Man 3D model

First entering service in September 2077 with the U.S. Army, the Fat Man is a shoulder mounted mini nuke launcher. The variant appearing in Fallout 4 has a slightly different appearance than in previous games. There is no longer a handle on the right side of the launcher, and instead two straight handles on the bottom. It also appears to be larger, with a rusted silver paint job.

Developers struggled to find a way to fire the heavy ammunition from a man-portable system far enough to keep the operator outside the blast radius, losing a lot of people in the process. Eventually, it was discovered that by placing a small sub-charge into the ammunition itself, similar to bullets being fired from a gun (or perhaps more accurately described as being along the lines of a recoilless launcher), the warheads could be launched a considerable distance without adding too much weight to the already heavy weapon.
(wiki)