We have the discussion around here quite often around the "you can build it shorter, but not longer" theory in regards to the specs on the Form 1.
During one of the discussions in the last couple months, somebody posted a letter/email from Gary S @ ATF stating the length was the best guess, and if it turned out shorter or longer you could simply let the ATF know. The terms are common to a lot of threads, so I'm not having any luck finding it by searching.
Anybody happen to have the link saved? I can't remember if it was a scan of the email/letter, or simply a copy & paste.