It's a pump-action 12-gauge shotgun with extremely high stopping power, capable of taking the player or an enemy down in a single shot. The weapon is preferred by gangsters as well as police officers, who will use it against the player once their wanted level reaches 3 stars.
The weapon can hold 8 shells and the player can carry an additional 48, for a total of 56 rounds. The shotgun's slow rate of fire is made up for with its high power, but it's very time-consuming to reload. It occupies a slot in the bottom of the weapons HUD along with other non-automatic two handed weapons.
They become available for sale at Harry's Gun Shop in Chapter 5, you can purchase one when you go to pick up the MG-42. They're also used by some of Siney Pen's men later in the same chapter. They don't become available at McClusky & Son until Chapter 7.
In both The Betrayal of Jimmy and Jimmy's Vendetta, you start off the game with a shotgun in your inventory. In Joe's Adventures they are available for purchase from the beginning of the DLC from Harry's shop or McClusky & Son.
You can find more real world information about shotguns on Wikipedia.
- While not available until Chapter 5, you can obtain one as early as Chapter 2 by the killing the attendant at any McClusky & Son's store; however, the only way to get more ammo before Chapter 5 is to kill him again. They are also occasionally carried by police officers, and one can be found in the back room of The Lone Star bar in Chapter 2.