You are right no doubt but as was stated previously it is hard to have constructive intent without owning the host weapon as well. Since a lightning link or drop-in auto sear is considered the machinegun by the ATF, and those items are serialized and transferred as full auto machine guns, no stipulations apply to them. M16 fire control parts by themselves, however, also require an M16 lower (third hole and low shelf) or a modified Ar15 receiver to be a machine gun. Owning an M16 FCG and also owning an unmodified Ar15 receiver that COULD be modified to accept M16 parts is also illegal according to the ATF because it shows constructive intent to use the M16 parts illegally. This is the situation that prompted the ATF to raid the Branch Davidian compound in Waco: they were reported to have bought M16 parts in addition to a large number of Ar15 receivers and the ATF started an investigation.
Think of the famous shoe string machine gun example. The ATF ruled a shoe string can be considered an illegal machine gun when (and only when) it is placed in a certain fashion on a host weapon. They didnt immediately define every shoe string in America to be a machine gun, they only stated that a shoe string added to a host weapon might be a machine gun if used improperly. In this same regard, an M16 auto sear, or an HK auto FCG, can be owned as long as you dont also own a receiver that is required to use that item (whether the receiver in question would be illegal or legal to own by itself).
In the case of the M16 parts, you would need a an M16 or modified AR15 receiver to use the FCG parts. In the case of an HK FCG pack, you would need a non-shelved HK receiver that would accept the non-clipped and pinned full auto FCG pack. In either case, though, just owning the trigger parts is not illegal.
