So my local shop just got the Class 3 and I downloaded the ATF's NFA handbook to make sure we are up to speed on everything and its actually a really good resource. I'm sure many people here are already familiar with it but for those who aren't, the link to download is below.
I did notice it is not current with 41F but otherwise some good info. Stuff on how NFA items are classified, how to do different forms, samples of the forms, what it takes to get a Class 2/3 SOT, detailed examples of parts that are considered illegal machine guns by themselves, etc. etc.
https://www.atf.gov/firearms/national-firearms-act-handbook
I did notice it is not current with 41F but otherwise some good info. Stuff on how NFA items are classified, how to do different forms, samples of the forms, what it takes to get a Class 2/3 SOT, detailed examples of parts that are considered illegal machine guns by themselves, etc. etc.
https://www.atf.gov/firearms/national-firearms-act-handbook