Fences are: Hemispherical outgrowths of the receiver of a double gun that mate with the breech ends of the barrels. The term derives from the flanges (or fences) in this position on a muzzle loading gun that were designed to protect the eyes of the shooter from sparks and escaping gasses.
Water table: The top of the bar of the action, the flat projection on the front of the receiver of a side-by-side gun, perpendicular to the standing breech. The cocking arms, hingepin and locking bolts are typically mounted inside the bar, below the watertable. The Table, or the Action Flat.
Cape gun: A two-barreled, side-by-side, shoulder-fired gun having one smoothbore shotgun barrel and one rifled barrel.
Those are from Hallowell's Firearms Dictionary, which is usually pretty handy, though I take exception to the terms watertable, and receiver, when referring to doubleguns, preferring the terms action flat, and action.
http://www.hallowellco.com/abbrevia.htm#F
