It is a weird set of ATF rules.. It confuses me truthfully. I bought an Hk MR556 and it has a fully auto hammer factory installed. I guess they got away with it because the barrel has a notch (easily removed) that won't allow the 416 BCG to be used in it. Truthfully I would say if they wanted to push it, the ATF could do what they wanted as far as prosecution. Who knows...
That's why I will leave this to the more knowledgeable members to answer.
" The strongest reason for the people to retain the right to
keep and bear arms is, as a last resort, to protect
themselves against tyranny in Government...." - Thomas Jefferson