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Described in-game as a one-of-a-kind pistol and a cut down hunting rifle, it has more in common in appearance with Deckard's combination laser/revolver in Blade Runner than any component of a larger rifle.
While there is only one of its kind in the first Fallout, they are slightly more common in the second game. It doesn't quite pack the punch of the gauss pistol, but it's cheaper, available earlier in-game and uses a far more common form of ammunition - the same .223 JHP as the common hunting rifle. That, the relatively boomy thud of its fire and the tendency to grant ridiculous criticals makes it the favourite pistol of many Fallout fans.
Interestingly, the in-game sprite for the .223 pistol is the same dual-grip machine pistol as the 10mm machine gun.
Unfortunately, once you hit the end-game areas, the .223 pistol becomes almost entirely useless due to its inability to pierce Power Armor or Advanced Power Armor beyond the occasional critical hit.
Similar to any other single-shot weapon, killing an enemy with a critical shot to the eyes mysteriously blows out half of their midsection, due to the limited design of enemy sprites.