Been there done that. You can finish the receiver complete before registering, and have 30 days from completion to do so (and no you CANNOT take it to the range before registering). They require pictures of both sides and from the top down showing the internal cavity. You must show the serial no., and it has to be a certain depth (about .010 if I remember correct) Also they will veto a serial no if it is offensive (like f### you) and you get to grind it out and start again. Other than that you can put in whatever you like for a serial no. Once they have approved it, they will issue you an frt # and you just register it online, and it can be registered as a receiver only, although they say once you put an upper on it to re register it, but if you are using multiple uppers their isn't much point. My AR 15 is a receiver only , and my AR10 is barreled. This was for receivers from scratch, but I doubt it is any differant for an 80%.
They never actually asked to see pictures of the serial numbers on the ar15(s), nor made reference to the depth of the engraving. Just wanted to see the FCG pocket to make sure it wouldn't fit an auto-sear. They did ask for the serial number, just not a pic of it.
It might have been different for the Ar10(s) since no one had made homemade ar10 lowers before.
I know that for businesses the specifics on what, where, and how deep the engraving is, is more specific.



















































