The Wyoming Law Review's Special Edition on the National Firearms Act is now available. Last fall, the Firearms Research Center at the University of Wyoming College of Law hosted a symposium on historical, statutory, and constitutional issues involving the NFA. The papers stemmed from that conference.
Here's the abstract from my contribution, The Power to Tax, The Second Amendment, and the Search for Which "Gangster Weapons" To Tax:
Congress does not have the power to ban firearms. The National Firearms Act (NFA) is based on the power of Congress to lay and collect taxes. In 1937, the Supreme Court upheld the NFA as purely a revenue measure. When it banned possession of machineguns in 1986, Congress undercut that constitutional basis. The Supreme Court has held that any ambiguities in the NFA must be read narrowly according to the rule of lenity.