According to this report in the Washington Post, the U.S. Court of Appeals for the D.C. Circuit has upheld the Bureau of Alcohol, Tobacco, Firearms and Explosive’s (ATF) authority to demand that gun dealers in New Mexico, and the three other southwestern border states, report to ATF “when anyone buys — within a five-day period — two or more semi-automatic weapons capable of accepting a detachable magazine and with a caliber greater than .22.”
The requirement is intended to help track or stop the sale of firearms to Mexican drug cartels (presumably, other than those that ATF allows to be sold to them).
The ruling can be found at National Shooting Sports Foundation v. Jones.