If the NFA is essentially tax legislation then sliding an amending clause - that reopens the Registry - into one of the various multiple volume tax bills would seem to be an effective way of progressing the issue.
This is great in theory, but it carries a huge inherent risk- shining a spotlight on the fact that the $200 NFA tax has never been adjusted for inflation. When it was enacted in 1934, the purpose of the tax was to make NFA firearms prohibitively expensive; $200 had about the same purchasing power that $4,000 has today.
I shudder at the thought of a Congressional floor debate about the "Machine Gun Tax"- an unfortunately snappy slogan- and the campaign commercials it's bound to spawn, particularly in moderate to left-leaning states. "My opponent, [insert name], voted to allow the purchase of brand new full-auto machine guns for a fee of only $200, less than it costs to buy a new iPad. As your Congressman, I will vote to raise the Machine Gun Tax and keep these lethal military weapons off the streets, away from our schools, and out of the hands of potential terrorists." (Footage rolls of children walking to school, interspersed with footage of firefights in Afghanistan and a car being cut to pieces at Knob Creek.)
Although I fundamentally agree with the premise that private ownership of machine guns deserves full 2A protection, IMHO suppressors and short-barreled rifles are far more useful to the average shooter than a machine gun, and I would greatly prefer to have these removed from the NFA (+1 BillM) before approaching the machine gun issue and risking throwing the baby out with the bathwater.