dakota.potts said:
It seems I remember being told that you should list the shortest barrel length you anticipate and then you can go up from there but it is recommended to update the ATF if you plan to put a shorter barrel on it
That's a common myth, but it's not correct.
There is one configuration listed on the Form 1 or Form 4: caliber, barrel length, and overall length. But you can change this configuration any time you want without notifying the ATF, and it doesn't make a difference if you go shorter or longer; once it's an SBR, barrel length is irrelevant.
However, the ATF wants you to notify them if you plan on permanently changing the configuration listed on the Form 1 or 4 (again, whether you're going longer or shorter doesn't matter). This means you should notify them if you no longer possess the parts required to put it back to its registered configuration.
For example: If I have an SBR registered with an 11.5" barrel chambered in 5.56, I can take that upper off, throw it in my parts bin, and put an 8" 300 Blackout upper on it instead (or any other upper). I don't ever have to put the 11.5" 5.56 upper back on, and I don't need to ever tell the ATF anything.
However, if I decide to sell the 11.5" 5.56 upper (and I don't have the parts required to assemble another 11.5" 5.56 upper) then I should notify the ATF of my SBR's new registered configuration.
Keep in mind that this notification requirement doesn't seem to have the force of law behind it; as far as I know it's just something the ATF requests.