SEC. 17. Any person discharging firearms within the limits of the city, without a lawful breastwork or battery for the protection of the citizens, shall be liable to a fine of not more than twenty-five dollars for every such offense. A breastwork or battery, for target shooting, to be deemed lawful, shall be a wall eighteen inches thick, six feet wide, and six feet high in the back, with side wings one foot thick, each extending two feet, increasing flaringly to the front, and six feet high, of adobes or mud, or its equivalent of other material.