There are some guns for which the upper receiver is the controlled part though. FALs, SCARs, etc consider the upper the controlled part, while the FNC is in th

There are some guns for which the upper receiver is the controlled part though. FALs, SCARs, etc consider the upper the controlled part, while the FNC is in the curious position of being sometimes upper, sometimes lower, all depending on where the manufacturer decided to stamp the serial number. In short, there's no logical reason that the lower is considered the firearm over the upper, except that's what the ATF defined it as... more reasons not to ever assume anything. :-)