Sec. 46.05. PROHIBITED WEAPONS. (a) A person commits an offense if he intentionally or knowingly possesses, manufactures, transports, repairs, or sells:
(1) an explosive weapon;
(2) a machine gun;
(3) a short-barrel firearm;
(4) a firearm silencer;
(c) It is a defense to prosecution under this section that the actor's possession was pursuant to registration pursuant to the National Firearms Act, as amended.