The Kneecapper

The Kneecapper is the unique version of the Sawed-Off Shotgun, featuring increased damage and faster reload time. It can be obtained by killing Ronald Laren, from Girdershade. It is repaired by ordinary Sawed-Off Shotguns.
 * In this commenter's opinion its the highest damage shotgun in the game, despite being 75 dmg and not 80 like the Terrible Shotgun. The reason is simple, the Keecapper is accurate, it has a large grouping, but its rather tight at mid ranges, unlike the Terrible Shotgun which can't hit the broad side of a barn, from inside the barn!
 * The downside to the Kneecapper however is its huge AP cost, you need 10 agi and an action boy to fire both chambers in VATS. To make up for this, target the chest using VATS, then hit B and pull the trigger, from most angles you'll be centered perfectly on the target and the trick doesn't cost you any AP, or force you to watch slow motion camera shots.

Trivia
The Kneecapper may take its name as a reference to Once Upon A Time In Mexico, where the hero, El Mariachi, blows off the knees of his enemy with a single sawn-off shotgun blast.