I mean, the first thing that comes to mind in the 80% parts. But really this can apply to an aluminum block with slide grooves in either side, for a 1911 or 2011 model.
The squishy term ‘readily’ is cooked up by ATF to cover whatever they want. It's definition is the definition of ambiguity.
“The squishy term ‘readily’ is cooked up by ATF to cover whatever they want”
Old aphorism: a competent and minimally equipped gunsmith can turn a VW Bug into a machine gun in 8 hours.
It seems nearly impossible to determine what would be an incomplete receiver, and what would be a block of metal. There would need to be a whole set of regulations or a law defining exactly how far along the part becomes an "incomplete receiver".
That seems like a critical issue, since even a licensed firearms manufacturer could end up shipping or having un-serialized incomplete receivers in existence during the manufacturing process. What happens if a firearms manufacturer decides to use an external CNC machine shop to do some work on their receivers before they finish the machining in house. Do they need to serialize the partially finished receivers?