I am talking about the form 4473. Some people are more comfortable by having that done but I'm not sure it makes a difference.

Do you have a link or PDF from the ATF document? Everything I have ever seen says once its a rifle its always a rifle. They ahd some problems with pistols sold with longer barrels and stocks claiming that once the stock was put on it was now a rifle and if it was coverts back to a pistol it was now considered an SBR.

It would be great if I was wrong. The make or used to some great conversion carbine kits for 1911's and Glocks.