The Bureau of Alcohol, Tobacco, Firearms and Explosives has proposed that it be given emergency authority for six months, beginning January 5, to require about 8,500 firearms dealers along the border with Mexico "to alert authorities when they sell within five consecutive business days two or more semi-automatic rifles greater than .22 caliber with detachable magazines."
The Washington Post story reporting on the BATFE proposal described that definition as being applicable to "so-called assault weapons," but it would also apply to many rifles that have never been labeled with that term.
The reporting requirement will apparently be imposed under the "authority" the BATFE has used in the past to demand reporting of other types of transactions from certain limited groups of dealers over the past 10 years, but the new proposal is far broader than any previous use of this authority.
Of course, there's no law today that prevents dealers from reporting suspicious transactions (or attempted transactions) to the BATFE, and dealers often do so. The BATFE is also free to inspect dealers' sales records--either for annual compliance inspections or during a criminal investigation.