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Gun Runners - the Vendortron sells one. Mitchell's house, there's a broken one on top of the Sunset Sarsaparilla crate that can be repaired if you have a Repair skill of 25. Found on various merchants and traders all throughout the Mojave Wasteland. Commonly found on White Legs tribals in the Honest Hearts add-on.
Clair's house - hidden on the floor underneath the shelf.
Andy Scabb carries a 9mm submachine gun. Vance's 9mm submachine gun, a unique variant of the 9mm SMG, used by "master criminal" Vance. 9mm SMG light bolt, increases rate of fire by 30%. 9mm SMG drums, +30 rounds per magazine. The 9mm SMG can fire 4245 standard rounds from full condition before breaking. In addition to losing much of its damage, the high spread of this weapon requires the player character to be in close quarters for all of the rounds fired to impact on the target. However, it shares the common weakness of most shotguns and automatic weapons in Fallout: New Vegas, in that much of its theoretical power is negated by the damage threshold system. With the light bolt modification, this weapon can do 200 damage-per-second.
The 9mm rounds doesn't deal quite as much damage as 10mm or 12.7mm rounds, but they're common and light, making it even easier to spray-and-pray. It's a compact weapon, with a telescoping one-piece wire stock. An automatic, air-cooled blowback-operated weapon firing from an open bolt, the 9mm submachine gun offers an exceptional rate of fire, capable of emptying its full 30-round magazine in under 3 seconds.