My (very limited) ATF experience has been that of nice people trapped in a huge beurocracy. I had a question about legality of something... asked the police, they referred me to the ATF. First number on the local ATF website didn't work. Second was a recording sending me to another number. That number got a local agent who had no idea how to answer my question and gave me a number to their firearms technology department in Virginia.
Called that, get a recording saying they no longer answer phone calls or take messages or emails, but to send them a snail-mail letter.
Surprisingly enough, I did get a phone call from the guy a few weeks later, and he was very helpful. Simple answer to what should have been a simple question, but apparently none of the local ATF agents or police knew that, and some were trying to make it unneccessarily complicated.
Oh, the question? What makes something a gun vs. a non-gun. I had been hoping I could modify a shotgun reciever so it could still cycle casings, but was incapable of firing, and then make that a legal non-gun prop to rent out to a theatre company. No dice- the whole once-a-gun, always-a-gun thing.
Some of the local folks were trying to tell me it was totally different for every model of firearm, though, which seemed a staggeringly inefficient way to make laws.