They are classified as AOW ( Any Other Weapon ). In short, if the gun does not currently have a "stamp", it is illegal.

http://en.wikipedia.org/wiki/National_Firearms_Act
http://www.atf.gov/firearms/faq/firearms-technology.html

If the gun in question does have a stamp, you can purchase it. However, you must complete a Form 4 http://www.atf.gov/forms/download/atf-f-5320-4.pdf . The transfer, if interstate, must go through a Class III FFL. Your state must allow possession of such a weapon. Your local police deptmartment also must approve.

In the end, because of all the paperwork, the market for these guns is small.

Pete

http://www.damascus-barrels.com/Movie.html