To the ATF, a machinegun is a machinegun AND/OR a combination of parts that will allow more than one shot to fire with one pull of the trigger, even if not reliably. So, a worn, malfunctioning semi can also be considered a machinegun.
So, let's go over this part by part.
There is no differnce in the M16 and AR15 bolts. The bolt carriers are different.
The bolt carrier: The M16 bolt carrier has a surface on the bottom rear which is meant to trip the autosear of a full auto version. This is ground away on the AR15 carrier so that even if a GI autosear, or a drop in autosear were added, there would be nothing to trip it. In addition, just in case someone tried to remove the disconnector and get the AR15 to fire full auto (which is barely possible with very soft primers), there is a ramp machined up toward the head of the firing pin. The idea is that the notch on the hammer will catch on the head of the firing pin and prevent the bolt from moving forward if the disconnector does not catch the hammer. Thus, an M16 bolt carrier alone, with the removal of the disconnector, and soft primered ammo, MAY allow a double.
Hammer: A notch is machined in the top front of the hammer (see above). The autosear spur is machined off. If an autosear were present, and a drop in autosear used, and a bolt carrier conversion used (that is a screw on autosear surface-see Bushmaster catalog, replaces part machined off AR15 bolt carrier) could make a full auto weapon. (Grinding the autosear spur off the rear of the hammer alone is enough to be a legal AR15 hammer)
Trigger: Slot in rear of M16 trigger could allow installation of full auto disconnector. This one part alone is iffy.
Disconnector: Can only be installed if there is a M16 trigger with slot in back. These two are also iffy, by themselves.
(Commercial AR15 triggers and disconnectors are made by shearing or grinding the tail off the disconnector, and welding up the slot at the rear of the trigger.)
Well, they have all sorts of theories for all of the parts. Yes, it stretches the imagination.