The length is not really important. What is important is the receiver is in the NFA registry as an SBR, and remains as such until a change of status. So what length upper/barrel is really irrelevant.
Note: The myth that an AR15 SBR is not an SBR when you put a 16" upper on it is simply not true. The current configuration may not be physically, and the ATF calls that "temporary" simply due to the modularity of the AR15. You cannot unilaterally change the status in the registry of the receiver by putting a longer barreled upper on it. As far as the ATF is concerned, it is an SBR, and these clowns that think they can sell such an receiver w/o a tax stamp are fools. That would require a change of status in the NFA registry.
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