A brief synopsis for those of us following along on or cell phones?
A nice slate of state AGs argue that the prospective rule change exceeds the Gun Control Act's statutory boundaries, and is thus straight up unconstitutional. They also get into the endless practical absurdities of the proposed rule's ramifications, and also mention the business-destroying aspects of it. Mainly, though, it's a satisfyingly thorough tutorial on how the proposed rule looks a lot more like a new LAW that congress isn't authoring or approving, and thus it's not going to withstand constitutional scrutiny in court.
Also, a decent history lesson on "receivers" and on the centuries long tradition of home-built guns and the fact those were explicitly preserved in the GCA, while this rule seeks to stomp that out of existence via executive fiat.
The AGs make a compelling case that this won't stand in court. The ATF, of course, is just doing what the administration says, knowing full well that the entire purpose of this is to make people who generally speaking are their political opponents miserable by any convenient method, this being one of them.