Full Auto? They are covered by the beautifully named "Firearm Owners Protection Act"
Only MG manufactured, and registered before May 19, 1986 can be owned by "civilians". (btw: Cops are civilians, even if they want to ignore it. Unless you are paid Military, accept the title.)
Simple enough reason. Not AO decision, but Uncle Ronnie. (He signed the stupid law, backed by our beautiful NRA!)