An elongated forend member is the principal component of the modular system rifle stock. The forend member can be injection molded or milled from an aluminum billet. The forend member has laterally spaced left and right side walls whose bottom edges are connected to a bottom wall to form an elongated chamber that extends substantially the length of the forend member. The elongated chamber can accommodate most of the receivers/actions of existing rifles and shotguns. The rear wall of the forend member has a protruding boss member or specifically shaped recess that allows either a slider or firearm stock to be secured thereto.