A shotgun, also called a "scattergun", is a firearm designed to shoot a multitude of projectiles with a single pull of the trigger. This makes it very effective at hunting game, greatly increasing the weapon's hit potential. However, the accuracy and power found at close range diminish rapidly as you get farther away from the target. Shotguns designed for military use are often called "combat shotguns".
Someone took the time to chop the last few inches off the barrel and stock of this shotgun. Now, the wide spread of this hand-cannon's short-barreled shots make it perfect for short-range crowd control.
The sawed-off shotgun is a break action, 12 gauge shotgun that fires both shells at once. Each shot delivers 14 pellets (instead of the typical 7), and while its damage-per-second is low, it inflicts good burst damage at close range.
This weapon, like all other shotguns, has a large weapons spread. In fact it has the largest spread of all weapons, so much so that in order to gain the high damage of its shot, you have to fire it at point blank range so that all the pellets impacts the intended target.
Furthermore, it is also considered an improved holdout weapon, and can be concealed and taken into weapon-free locations so long as your sneakskill is 50 or greater. A sawed-off shotgun at full condition can discharge 396 times (792 shells) before breaking. This amount is exactly the same amount as its unique variant.
Big Boomer, a a significantly stronger and more accurate unique variant.
Sloan - In the worker barracks in an easy-locked cabinet.
Behind the scenes
The sawed-off shotgun, as well as its Fallout 3 counterpart, is almost visually identical to the shotgun used by Max in Mad Max 2: The Road Warrior. This is just one of the many Mad Max references in the Fallout series.
There is a weapon entry for a sawed-off shotgun that is set up to use magical companion ammo, but it isn't used anywhere in the game. The entry is positioned after all of the other companion weapons, like Veronica's 10mm pistol, or Boone's scoped hunting rifle, suggesting that it was to be the ranged weapon for a cut companion (potentially Ulysses).
PCPlayStation 3Xbox 360 The sawed-off is set up to produce a shell casing after firing, like the majority of semi-automatic weapons with an ejection port, so each shot technically produces three empty 12 gauge hulls - one flies out of the gun nonsensically after firing, and the expected two are removed when reloading.