Quote Originally Posted by RCCrawler View Post
Not that I don't believe you, but can anyone show where that is published?

If they don't care the actual length then why bother even asking? Just let us fill out a form saying SBR.
There are no laws saying you CAN do things - just laws saying what you can't do.

The NFA defines an SBR as a rifle with a barrel less than 16" - and any length is good to go on an SBR.

You are required to put a length because you're the maker, and you're supposed to put down the initial build configuration. After that, there's no laws forbidding you from ever changing it. The back of the Form 1 does ask you to write the ATF and notify them of any permanent changes - but there is no statute requiring you to do that, either.

If you absolutely must have a written answer, write the ATF Firearms Technology branch and they'll send you a reply.