Are you certain this is the case or can anyone else confirm this is true?
I have a 300blk SBR and would like to change it to a .22lr SBR and would probably do it if I only have to send a letter of notification.
The Form 1 is an application to make an NFA weapon. It is not a form to show all possible lengths, calibers or current configuration. Once you initially made your SBR in the caliber/length that you specified on the Form 1, you may change it all you want. You do not have to have permission for other calibers/lengths. ATF requests that you notify them of any permanent changes, but it is not required.