From the Document: "On December 18, 2020, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) published guidance in the 'Federal Register' for public comment that indicated that it was preparing to reclassify certain 'heavier, larger handguns (pistols) equipped with stabilizing braces' as more stringently regulated 'short-barreled rifles'. Such a reclassification would have retroactively triggered the more extensive paperwork and background check requirements of the 1934 National Firearms Act (NFA), and required registration of the owner and firearm with ATF. On December 23, 2020, however, ATF withdrew its guidance, pending further Department of Justice review."
CRS In Focus, IF11763
Congressional Research Service: https://crsreports.congress.gov/