Update ATF Form 1 with changing stock on SBR?
I have a SBR registered AR lower. I would like to change the stock (currently a Magpul CTR) to a Magpul UBR (if they ever become available again). Obviously, this will change the overall length originally listed on my Form 1. What do I need to do? Send a letter to ATF with the new overall length? If so, will they send me any type of reply? Do I have to wait on a reply before installing the new stock? Thanks for any help you all can provide.