If a barrel is a pressure bearing part as a component of the firearm it is fitted to how does it cease to be a none FAC item when you take it off?

What you are saying is that when I handed in my .357" Smith & Wesson when they were banned I could have taken the barrel off and kept it?
Or, when you scrap a gun you can saw off the barrel in front of the chamber and keep it?

I'm interested in your comments.