The ATF no longer allows for the use of Multical or Multi for the caliber of your NFA firearms. This directly effects that caliber that is used on your ATF application paperwork. This can also effect which caliber is required to be engraved on your ATF Form 1 NFA firearms - Form 1 silencers, etc. When you are applying for an NFA firearm that is capable of using multiple different calibers by way of, using a different upper receiver, a silencer that is capable of multiple calibers, etc, you will need to designate one caliber on your ATF application. For silencers generally speaking this is the largest caliber that the silencer can use. For SBR’s this would be up to you to decide the main upper receiver caliber that is going to be used on your SBR, if applicable for your SBR. If your SBR is only capable of using one caliber, then you would simply use that caliber on your application. You can change out the upper receivers for your SBR and the ATF requests that you notify them of the additional configurations. This information is used to update the ATF’s NFRTR. You can use our, "Changing Your SBR Upper with a Different Caliber Than the Registered Lower Generator" here for help. If and when you decide to sell your SBR, the original upper receiver would need to be installed back onto your SBR. In summation, if you are deciding to apply for a tax stamp for NFA firearm(s) that use multiple calibers, you will need to designate the caliber that the NFA firearm will be using and the Multical or Multi designation is no longer acceptable.