Regarding faxes, I know it's customary and acceptable to send some documents
via fax to the ATF, specifically 5320.20s for the Knob Creek shoot. Here's some more info regarding the ATF ruling regarding faxes :
Regarding possession of an NFA weapon, several quotes from James Bardwell's NFA FAQs
"It is illegal for anyone to have possession of an NFA weapon that
is not registered to them in the NFA Registry."
"While it is illegal for anyone to have possession of an NFA firearm that is not registered to them, ATF haas carved out an
exception for getting the guns repaired."
and from ATF Online
"Possession of an unregistered NFA firearm is a violation of Federal law and subjects the possessor to possible criminal prosecution and the seizure and forfeiture of the firearm."
If you're driving with a NFA weapon that is not registered to you - i.e. in possession of that weapon - you're violating the law. Regardless of the paperwork you may have, unless it's a corporate NFA weapon and the paperwork shows you are an authorized officer to possess the weapon.
Moving Registered NFA Firearms Interstate
Individuals other than qualified Federal firearms licensees must obtain permission from ATF prior to temporarily or permanently moving a machine-gun, short-barreled shotgun, short-barreled rifle, or destructive device interstate. To obtain permission to move these firearms, please submit to ATF either a letter requesting permission to move them or an ATF Form 5320.20, Application to Transport Interstate or Temporarily Export Certain NFA Firearms. This form is available at the ATF Internet website or may be obtained by contacting the NFA Branch. If you are submitting the request by letter, please include the:
• Name of registrant;
• The firearm(s);
• The current location of the firearm(s);
• The location to which the firearm(s) will be transported;
• The date(s) and means of transportation (car, plane, boat, etc.); and
• The reason the firearm is being moved.
Please also indicate in the letter whether: (1) the move is temporary or permanent; (2) the move will involve a transfer of the title; and (3) whether the possession of the firearm will violate local or State law at the destination. The ATF Form 5320.20 includes these requirements. Requests for interstate transportation will be denied by ATF if possession of the firearm at the destination will violate State or local law.
Registrants do not have to obtain permission from ATF to move a silencer or “any other weapon” interstate. However, if a registrant’s address for these types of firearms changes after registration with ATF, please notify the NFA Branch in writing of the new address, so the NFRTR can reflect the registrant’s correct address.