It's a large caliber revolver with an extra long barrel.
When carried, it goes into a heavy weapon slot. When running the player will either store it on his back, or hold it with two hands (one hand holding the barrel).
It's an easter egg for the revolver used by the Joker in the 1989 movie "Batman".
Aiming and firing takes about 4 seconds, because the lack of a stock makes it hard to hold a long-barreled weapon straight and because the recoil is very powerful.
But if the weapon is aimed well enough, it can 1-hit-kill a jet fighter, or any unarmored vehicle. Hitting the engine can also partly disable the engine of some lightly armored scout cars.
It can fire 6 shots before needing to be reloaded.
The aiming is hard to master, but once you get used to it, it can be a competitor for some missile launchers.