NFA Gun Trust Lawyer Blog

There have been many discussions on this and it appears that as of July 2010 the answer is:
Yes, and you will not be required to again register the firearm before replacing the short barrel. ATF recommends written notification to the NFA Branch when a firearm’s configuration is permanently changed or removed from the purview of the NFA.

If I put my original barrel back on my SBR or SBS, can I take it across state lines without using an ATF Form 5320.20?

If you no longer own the barrel, you can do this, but if you still own or possess the short barrel, the item is still restricted by the NFA unless you remove the item from the NFA by contacting the ATF.

If you want to make a permanent change to the SBR or SBS, it should be removed from the NFA and will not then be regulated under the NFA. If the owner maintains possession or control over the short barrel riffle or shotgun they should be careful about constructive possession of an SBR or SBS. While there is no requirement to notify the ATF of the transfer of an item which has been removed from the NFA, the ATF does recommend that you 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.

Until the firearms is removed from the NFA registry, it will have the same restrictions regardless of the current barrel length. You can not put a longer barrel on to bring to a state where a short barrel is permitted or avoid the necessity of a 5320.20 to cross state lines. Once a SBR or SBS it will always be a SBR or SBS until the item configuration is changed and the gun is removed from the registry. If removed, you can not the original Form 1 to change it back to an SBR or SBS at a later date

If you still own or have control over the barrel or parts required to assemble the SBR or SBS, the firearm would still be subject to NFA transfer and possession regulations. ATF recommends contacting State law enforcement officials to ensure compliance with state and local law. (In Florida and some other states, possession of a barrel and receiver which could be made in a SBR or SBS is a crime unless the firearm is registered as a SBR or SBS under the NFA.)

If you do not have the barrel the firearm is no longer a SBR or SBS.

Under the NFA the serialized receiver is what is recorded for registration in the National Firearms Registration and Transfer Record (NFRTR)