to make a shotgun that has the barrel length of less then 18 inches, it's called a SHORT BARREL SHOTGUN (SBS).
You register it on the FORM 1, you just put SBS in the TYPE OF WEAPON box. and if you do it on a trust, then you just need to send 2 FORM 1s, a COPY of your trust declaration along with the exhibit/schedule,
DO NOT SEND THE ORIGINAL TRUST, and form 5330.20 - Certificate of Compliance With 18 U.S.C. 922(g)(5)(B) (US citizen form), and 200 bucks. mail it all off to the NFA branch in W.Va, address is on top of the top of the form 1, and that's it
you can get the FORM 1s and form 5330-20 from the ATF pubs center.
http://www.atf.gov/dcof/index.htm
I would order 20 ea of the forms. SBRs are kinda addictive, so if you have a hankering for a SBR later on, you will already have the forms.
You already paid for that service of printing them up and postage, by way of taxes, so why not utlilize it and get the froms mailed to your house,? takes less then a week for delivery after you submitt the request