Thank you all, especially Edwardian, for your posts on this matter.
Sadly, I am still a little nervous even though I am sure we are in the right. I maintain a very healthy caution in dealing with another countries' authorities although I could argue my corner in the UK without breaking sweat!
It will not stop me coming over but it will colour my decisions on moving around the USA with my stock until I see something in writing from ATF.
They may not be able to make law but they could make my life very uncomfortable.