Yeah best I can figure is the weapon is in the Power Plant... and I am NOT going back in there for a while.
There is an assault rifle in a wall cabinet right inside the door though.
Also there is a bait shop up river from the Jefferson Memorial which has a water fountain, good place to gain health back cheap.
Someone in the game development loves T-33s or better P-80s.
Land mines are your friend, and so are kill zones with land mines.
Sniper rifle does suck.. what is up with that.
Oh for the new guys, disarming the bomb in Megaton gets you a house with lockers, bed, and even a robot butler.