So many answers .... mattnh gets the cupie doll for reading the book or at least the form instructions. With an avatar like that, who can be surprised?
Crossing state lines with a MG, DD, SBR, or SBS is prohibited. Exemptions are licensees and those who get permission (payment of tax or tax free transfer is of no consequence). See 18 USC 922(a)(4) or 27 CFR 478.28. Regulation is more strict - not supposed to be able to override a statute with a regulation, but things happen (like bump stocks).
Several years ago there were a few internet based firearm law experts that went to the mat claiming registration determined classification so if you registered a BB gun as a DD it became a DD... or more specifically argued, a firearm receiver registered as a DD/SBS/SBR became a DD/SBS/SBR. This theory differed from all applicable statutes and regulations as the claimed "optional registration" does not exist - what CAN be registered MUST be registered so if a stripped receiver meets the definition of SBR, all receivers must be registered, but arguing with experts once they have dug in doesn't work. Ironically, one of the people they abused the most for his conflicting comments was a real world gun lawyer who made his living at what they were attempting to do. Interestingly enough, around that time (2009-2010) ATF published the SBS/SBR FAQ (may as well say DD also - same concepts apply) which did not go over well with the experts, but for those not emotionally invested in being subject to even more government control, it was welcome guidance from a known source. It's attached for your reading pleasure.
On an unrelated side note, the "he who builds it registers it" is covered in Q2. Common internet guidance is you can register something then get someone else to actually build it and transfer back paperless. That is also in error.
Edited by The Lone Ranger, 29 August 2019 - 06:39 PM.