I had copied this from a FAQ document but can't find it on the ATF site at the moment, so I'm a little uncomfortable that maybe ATF has changed their minds. This is what I had from a couple of years ago:
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.
Question--control as in have it with you or have it at home?
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.
https://www.atf.gov/firearms/docs/at...ter-2/download
Section 2.5 Removal of firearms from the scope of the NFA by modification/elimination of components. Firearms, except machineguns and silencers, that are subject to the NFA fall within the various definitions due to specific features.
If the particular feature that causes a firearm to be regulated by the NFA is eliminated or modified, the resulting weapon is no longer an NFA weapon. This is a direct quote from the document above.