Unlike a machine gun, a short barrel rifle can be "un-NFA'ed." BATFE/NFA has warned me (on my registered multi-caliber, multi-length lowers) to always maintain at least one short barrel dedicated to that platform or else mine will not be considered a SBR.
Q: If I remove the short barrel from the registered SBR or SBS, is the receiver still subject to NFA transfer and possession regulations?
If the possessor retains control over the barrel or other 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.
Q: If I remove the short barrel from my SBR or SBS, may I move the firearm across state lines without the submission of ATF Form 5320.20, Application to Transport or to Temporarily Export Certain Firearms?
If the registrant retains control over the parts required to assemble the SBR or SBS, the firearm is still be subject to all requirements of the NFA. ATF recommends contacting law enforcement officials in the destination state to ensure compliance with state and local law.