The Vengeance: In the Deathclaw Sanctuary (northwest corner of the world map), follow the caves down and be prepared for many a Deathclaw encounter. In the southeastern corner of the caves is a pool of blood under a column of rock. In this pool there is a heap of gore with the Vengeance, an overpowered Laser Gatling Gun.

Waser Wifle: After getting access to Little Lamplight, you will find a child named "Biwwy" who will sell you the Waser Wifle, an augmented Laser Rifle, for 500 caps. If you have the Child at Heart Perk, you can get it for free (choose "You said you didn’t need it, why not just give it to me for free?").

Abe Lincoln’s Repeater Rifle: Enter the Museum of Natural History and go to the lower levels. In the Museum Offices, there is a central room with a gun case. Lincoln’s Repeater Rifle is here, a strong Hunting Rifle that uses .44 rounds.

The Kneecapper: In Girdershade, in the southwest corner of the world map, the town letch, Ronald, has a Sawed-Off Shotgun named the Kneecapper. You’ll have to kill him to get it, passively (via a pissed off radscorpion) or actively (a shot to the head).

Terrible Shotgun: In Evergreen Mills, in the southwest corner of the world map, you will find a heavily protected raider hideout, including a Super Mutant Behemoth. Inside the center building is a series of caves where the raiders have set up a brothel and a bar. One of these raiders won’t attack you, and his name is Smiling Jack. He’s a trader, but you should trade him a bullet to the head for his Terrible Shotgun, a better Combat Shotgun.

The Firelance: One random event while exploring the capitol wastes is an inexplicable explosion. Head to this area to find the Firelance, a souped-up Alien Blaster and some scattered ammo (which glows and is easier to find at night).

Jack the Ripper: This upgraded Ripper can be found at the lower-middle of the S-shaped map of the Deathclaw Sanctuary. It’s on a dead Enclave Officer in a pile of bodies, next to a giant ant.

Sydney's 10mm "Ultra" SMG:Inside or just the outside Museum of History in Downtown DC you will find a Super Mutant-fighting Ghoul named Sydney. She asks for help defending the area from Super Mutants. You can help her and if she dies, take her "Ultra" SMG -- or just kill her and steal it.

A3-21's Plasma Rifle: In the "Replicated Man" sidequest, do not to turn Harkness in to Dr. Zimmer. Instead speak with Harkness and chose to unlock his repressed memories. Once you do this, speak with him again and tell him that he should not kill Dr. Zimmer, but instead tell him to run away. He will thank you and give you his old (albeit more-powerful) Plasma Rifle! You may be able to kill him for it too.

Smugglers End: Once you gain access to the Brotherhood Citadel you can take, search or lockpick anything that you want. When you are in the Solar room you can lockpick the Elder Lyons safe (if your lockpick skill is high enough) and find 300 caps, other misc. items and Smugglers End, a powerful laser pistol.

O1' Painless rifle: In the Republic of Dave, in the northeast corner of the world map, kill Dave to get his key and access to his safe. Inside is the Ol' Painless Rifle.

Xuanlong Assault Rifle: This powerful Chinese Assault Rifle is located on the body of Jiggs in the diner ner the Jury Street Metro Station. This location can be discovered via the Museum of Technology mini quest, Jiggs' Loot.

Reservist's Rifle: This is dropped by the Drifter in Dickerson Tabernacle Chapel (upper west of world map). It is an augmented sniper rifle.

Victory Rifle: This augmented Sniper Rifle is in a Very Hard locker in the Sniper Shack to the west of Rockbreaker's Last Gas (Middle west of worldmap).

Protectron's Gaze: The Mechanist has this augmented laser pistol. Get it by giving him the Antagonizer's costume. *** this results in negative Karma.

Miss Launcher: This powerful missile launcher also is 5 WG less in weight than its normal counterpart. Find it in the storage room in the lower level of Fort Independence.

Zhu-Rong v418 Chinese Pistol: Found in the east wing of L.O.B. Enterprises in a Very Hard Locked case on a desk upstairs, the Zhu-Rong is a pistol that will ignite flammable substances.

Blackhawk: The augmented Scoped .44 Magnum can be obtained from Agatha after completing both Agatha's song and an additional Sheet Music fetch quest (in the Springvale school). You must request a reward from here in the dialog tree for this.

The Burnmaster: In the very bottom of Falls Church/Mason St. Metro you will find a radiated ghoul camp; here you will find the Burnmaster, an improved Flamer.
Stabhappy: Stabhappy is an upgraded Combat Knife in the "Raid Shack", a house of Raiders. Just head directly East from the Bethesda Ruins and you will see a fenced in house called the Raid Shack. Kill the Raiders inside and one of them carries Stabhappy!

Fisto!: In the MDPL-13 power station, enter the door on the right. You will be in an office and on the table in the center of the room is Fisto! the upgrade to the Power Glove. Beware of Ghouls.