When does a firearm stop being a firearm?
Let's say I were to take a 100% AR-15 lower and mill out the internals so that they no longer take AR-15 FCG parts, does it cease to be a firearm? At least for transfer purposes.
What if I were to take an 80% lower and mill the whole FCG pocket out of spec (too wide, too short, too deep). Is it not considered a firearm? Especially since it did not start as one.