Originally Posted by PoFF
A reciever for an AR-15 type firearm is the lower part of the assembly, that bears the serial number. ANY other part of the firearm is not considered a firearm and is not subject to control, including the upper reciever. That's for the real-steel, I'd guess the same would apply for airsoft. IMO a gearbox doesn't even closely ressemble an M-16 lower reciever.
I beg to differ with you on this. Any part of a restricted firearm is subject to control, especially a barreled upper for an AR-15. Its legal to cross with it if you have the correct documentation, but just Joe Schmoe sending a 20" DPMS upper in the mail will have that item seized 99.999% of the time. In the US, any part that attaches to it is controlled, and needs proper exportation documents (if its not on the "Do Not Export" list)
Just from my experience importing real-steel parts from the US.