Quoted:
If you install a 16"+ barrel and the oal is 26"+, your sbr is considered a standard title 1 weapon. You can still swap back to an sbr, because you have the approved form 1, but it is not considered an sbr unless it is in the actual sbr configuration.
If you permanently return the gun to a title 1 configuration, you need to notify the atf to amend the registry to note this. This is the easiest way to sell and sbr without doing a form 4. Just sell the upper separately and sell the lower as a title 1 lower.
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you're actually not required to notify them of anything... once you add that 16" barreled upper your rifle no longer falls under the purview of the NFA
from the ATF FAQ
Q: Does the installation of a barrel over 16 inches in length (SBR) or 18 inches in length (SBS) remove the firearm from the purview of the NFA? If so, is this considered a permanent change?
Installation of a barrel greater than 16 inches in length (SBR) or 18 inches in length (SBS) will remove the firearm from the purview of the NFA provided the registrant does not maintain control over the parts necessary to reconfigure the firearm as a SBR or SBS.
Q: Is it necessary to send notification to ATF and receive acknowledgement that the SBR or SBS has been removed from the purview of the NFA before it may be sold as a GCA firearm?
There is no requirement for the possessor of a registered NFA firearm to notify ATF that the firearm has been removed from the purview of the NFA. However, ATF recommends the possessor notify the NFA Branch of such changes in writing so that the possessor is not mistakenly identified as the owner if the firearm is later used in a crime. If, at the time of transfer, the firearm does not meet the definition of a SBR, it should be transferred without filing the NFA transfer application and without payment of the transfer tax.
Q: May I transfer the receiver of a short-barrel rifle or shotgun to an FFL or to an individual as I would any GCA firearm?
Yes. A weapon that does not meet the definition of a NFA “firearm” is not subject to the NFA and a possessor or transferor needn’t comply with NFA requirements. The firearm is considered a GCA firearm and may be transferred under the provisions of that law.
Q: Who is responsible for notifying the NFA Branch when I transfer the GCA firearm to a FFL or another individual?
There is no requirement that the transferor or transferee of a GCA firearm notify the NFA branch of a transfer or that either party determine whether the firearm was previously registered under the NFA. There is no also no requirement for the registrant or possessor of a NFA firearm to notify ATF of the removal of features that caused the firearm to be subject to the NFA; however, ATF recommends the owner notify the NFA Branch in writing if a firearm is permanently removed from the NFA.