http://www.atf.gov/forms/download/atf-f-5330-3d.pdf

I've jumped thru this hoop a number of times to take my shotguns down into the US for The Southern SxS Classic in Sanford NC. This last time, I included my e-mail address on the application forms, and I received back an e-mail "Import Permit". Not sure if I will be receiving a snail-mail copy.
Real ALL the fine print and submit EARLY.