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 can list them in 4h).
Please note that all barrels must be short, if you want to use a barrel that would not be classified as a SBR or SBS, you must remove the item from the NFA and then you would have to purchase a new tax stamp to make it a SBR or SBS in the future.
