Regarding the AR-15 type firearm...

Can we get a synopsis?

Yes - The ATF in their 365 page document of proposed rules for "ghost guns", admits that the AR-15 is one of the most popular sporting rifles in the United States.

From the public filing-
"In the past few years, some courts have treated the regulatory definitionof "Firearm fame or receiver" as inflexible when applied to the power portion of the AR-15 type rifle, one of themost popular firearms in the United States."

By this admission ATF/DOJ have basically conceded in a public filing that AR-15 Ban's in places like CA, NY, MA, MD, etc, are unconstitutional based on previous SCOTUS precedent in the Heller and Caetano decisions.
Those cases demonstrate as a matter of law that the AR-15 is the type of weapon protected by the 2nd amendment, because it's typically possessed by law abiding Americans for lawful purposes.

So if the SCOTUS takes an AR-15 ban case out of one of the ban states, this admission will play heavily in a positive decision for the elimination of State level bans.

Whoever wrote that part of the public filing is either A, Already retired from ATF, or B, is now stationed at the ATF Research Center in Tule Greenland.