[larrytown]

f the federal government wishes to ban "full auto functionality" (whatever that is) rather than "machine guns", the federal government should pass a new law, not ignore and/or magically reinterpret existing law...

Indeed. Adding a consideration for rate-of-fire to the definition would make it quite clear.

[Who is John Galt?]

Adding a consideration for rate-of-fire to the definition would make it quite clear.

I think your point is critical: a law that is "clear" does not promote 'word parsing' by citizens OR bureaucrats. But adding 'rate of fire' could possibly create more confusion, rather than clarify things. Would it be rate of fire as designed, or as tested (with worn parts)? Over what period of time (2 seconds? 5 seconds?), with what magazine, what ammunition, and in a machine rest or hand-held (if the latter, by whom)? At what temperature (above or below freezing?) would the test be conducted, and at what humidity levels (at or below 100%)? How would the firearm be lubed, and with what? Any number of factors could influence rate of fire; in comparison, the existing standard (essentially 'one shot per pull of trigger') seems more simple, and reasonably clear - even though it has obviously caused some difficulties for law enforcement & the judicial system...