The Trace on 18th September carried an fascinating article on the smuggling of guns from the US to Mexico. It says that whilst federal law prohibits the ATF and local law enforcement agencies from releasing the results of crime gun traces, firearm trafficking patterns are hidden from public view. However, it says, leaked records reviewed by The Trace tell 2 clear stories: high-volume gun traffickers often depend on a single retail gun dealer for most of their wares, and rarely do those gun sellers face consequences. The data reviewed by The Trace represents only a slim fraction of the total number of US-purchased guns recovered by Mexican authorities. Every year, the ATF conducts 10,000 to 20,000 traces of guns found at Mexican crime scenes, 70% of which were originally sold by American retailers. One expert is quoted as saying that a relatively small number of retailers in this country are disproportionately involved in selling guns later used in firearm-related violent crime in Mexico. Under US federal law, gun dealers only have to notify the ATF of very specific purchasing patterns: namely, if a single person buys more than 1 handgun from the same licensed dealer within 5 business days. In states along the south-west border — Texas, New Mexico, Arizona, and California — dealers also have to alert the ATF if they sell more than 1 rifle or shotgun to the same person within 5 business days.
https://www.thetrace.org/2018/09/atf-documents-crime-guns-us-mexico-trafficking/