Written by Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE), Chief of the Firearms Technology Branch
...M16 bolt carriers are not designed and intended solely and exclusively for use in converting a weapon into a machinegun and are not any combination of parts from which a machinegun can be assembled. Further, an M16 bolt carrier is not a firearm as defined in the GCA or a machinegun as defined in the NFA. An M16 bolt carrier is simply a machinegun part and as such its domestic sale and possession is unregulated under the Federal firearms laws. It is not unlawful to utilize a M16 machinegun bolt carrier in a semiautomatic AR15 type rifle.

Prosecution for something like this would stir up such a shit storm someone on this board would know about it and the NRA would be on that like stink on shit.