If you want to use multiple caliber barrels on a SBR or SBS there here are some guidelines that will help you fill out your ATF Form 1, which are also found on our updated How to fill out a Form 1 for a Trust Page.
4c should contain the caliber or gage of the firearm. Only one is acceptable. ( Multi is not acceptable, you can only list one, if you have additional calibers you want to list you should attach additional configurations in a letter attached to your Form 1 stating the caliber, barrel length, and overall length as related to the firearm listed on the Form 1.)
Previously ATF accepted them by being listed in 4h on the Form 1 but no longer accepts this.)
Please note if you want to use a barrel that would not be classified as a SBR or SBS, the ATF has written that the temporary change is permitted and allows one to travel interstate without an ATF 5320.20 as long as the short barrel is left in your home state. You could also travel with the receiver alone as long as the short barrel was left in your home state.
In the same corespondence ATF states that as long as you still own the short barrel, the receiver is still conssidered an SBR.
You may remove the SBR or SBS from the NFA registry only if you no longer have the short barrel. If you remove the SBR or SBS from the NFA registry and wanted to make it an SBR or SBS at a later date you would have to purchase a new tax stamp.
On one hand the ATF states that as long as you have a short barrel it is still an SBR or SBS but then state that if you want to travel across state lines, it will not be require the ATF 5320.20 preapproval that is required for a SBR or SBS as long as you leave the short barrel in your home state. This seems like an area where the ATF may change their interpreation in the future so you might play it safe and always get an ATF 5320.20 prior to crossing state lines with receiver that is a SBR or SBS.
Either way, you may want to download a copy of this ATF Letter and keep it with you or available to support your position if it is ever needed.
ATF Letter on Transport a Title II as a Title 1 across state lines.pdf