ATF ruling on autosears and receivers
ATF Ruling 2008-1 is here, in pdf. On its face, it changes the definition of which receiver half is legally the "firearm" for an FNC rifle.
What is of concern is part of its reasoning. There are autosears for the FNC; if installed these allow it to fire full-auto. Autosears have been held to require NFA registration, as a device designed to allow automatic fire; each one is serial numbered, registered, and traditionally treated as the registered machine gun. To the best of my knowledge, it was never argued that putting one into a semiauto made the remainder of the semiauto a machinegun with a separate registration requirement. But this Ruling's reasoning is that since you would have to drill and mill a semiauto FNC receiver to accomodate the autosear, doing so would make the receiver a full-auto receiver, and violate the 86 ban.