Nothing contained in this subchapter prohibits or interferes with:
(1) The manufacture for and sale of machine guns to the military forces or the peace officers of the United States or of any political subdivision of the United States, or the transportation required for that purpose;
(2) The possession of a machine gun for scientific purpose, or the possession of a machine gun not usable as a weapon and possessed as a curiosity, ornament, or keepsake; or
(3) The possession of a machine gun other than one adapted to use pistol cartridges of 30 (.30 in. or 7.63 mm.) or larger caliber, for a purpose manifestly not aggressive or offensive.