The point being, only NFA branch employees have even the slightest idea how the NFA works. Their opinion on the phone is nothing more that just that.
IF you want a legal opinion on an NFA firearm, write to the Firearms & Technology Branch of BATF&E. Be very detailed in your question and you will get a written response that will hold up in court.
Unless you consult an Attorney that is a specialist in NFA Firearms cases, you are wasting your time and money.
The National Firearms Act of 1934 is pretty easy to read and understand.
Is sounds like you are trying to "PLAY" or find a loophole in the NFA.... I highly recommend against that !