The Kneecapper

The Kneecapper is a unique Sawed-Off Shotgun, featuring increased damage and faster reload time.

Location

 * It can be obtained from Ronald Laren from Girdershade.

Strategy

 * To make up for its huge AP cost, target enemies center mass in V.A.T.S., then cancel out and pull the trigger, from most angles you'll be centered perfectly on target and the trick doesn't cost you any AP, or force you to watch slow motion camera deaths.

Obtaining the gun from Ronald Laren

 * When talking Ronald into venturing for Quantums on his own (Black Widow Perk required) he usually dies on the way and his corpse can be looted near or in the Nuka-Cola Plant. I chased him out of the gate and watched him immediately pick a fight with a Yao Guai, I ran up behind to watch it take 3/4 dmg from a load of buckshot, then retaliate with a huge overhand claw that decapitated him - I tossed a frag grenade and looted the Kneecapper.
 * Pickpocketing Ronald's shotgun ammunition, and then leaving, saving, reloading and returning to the area will cause his weapon to appear in his inventory, making it possible to steal without any violence and only minimal karma loss.
 * It's actually relatively easy to get this weapon without killing Ronald. Attack him with a weak weapon (so that you don't kill him), then shoot the gun from his hands.  Put the weapon away and he will stop attacking you (he will fight with his fists otherwise).  Now use the grab button to move the gun so that you can take it while he's not facing you, then just take it.  This counts as a theft and thus carries a small karma loss, but the loss is much less than killing him for the weapon, and as long as he doesn't see you while you're taking it, he won't become hostile again.
 * One more way, anytime there is someone you must kill to acquire an item, (in this case you don't have to, but if you want to) but killing them will result in negative karma, simply bring a follower with you and attack the target unarmed or in the leg with your pistol. The target character will become hostile, causing your follower to kill them so that you don't get any negative karma. (This has been tested, except I caused Jericho to clear out the entire lobby floor of Tenpenny Tower, then looted the shopkeepers for their inventory keys.)
 * Additionally with a little luck the mysterious stranger perk can be used here. Just target the kneecapper in VATS and if you luck out the stranger will come in and kill Ronald for you with zero karma affect.

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.