§ 1. . . . [E]very person, firm or corporation, who within the State of California possesses any firearm of the kind commonly known as a machine gun shall be guilty of a public offense and upon conviction thereof shall be punished by imprisonment in the state prison not to exceed three years or by a fine not to exceed five thousand dollars or by both such fine and imprisonment. Provided, however that nothing in this act shall prohibit police departments and members thereof, sheriffs, and city marshals or the military or naval forces of this state or of the United States from possessing such firearms for official use in the discharge of their duties. § 2. The term machine gun as used in this act shall be construed to apply to and include all firearms known as machine rifles, machine guns or submachine guns capable of discharging automatically and continuously loaded ammunition of any caliber in which the ammunition is fed to such gun from or by means of clips, disks, drums, belts or other separable mechanical device.