A shotgun, also called a "scattergun", is a firearm designed to shoot a multitude of lead or steel balls of varying sizes. 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".
The Kneecapper deals much more damage and is significantly more accurate than the regular sawed-off. It is not as accurate as the combat shotgun, but deals more damage per shot, though it must be reloaded six times as often because it only holds 2 shells. The Kneecapper deals damage through raw power alone, since it does not deal any critical hit damage.
It is possible to obtain two Kneecappers. The first by pickpocketing it from Ronald Laren while he is in Girdershade. The second by collecting another from his corpse if he is sent to the Nuka-Cola Plant using dialog from the Black Widow perk.