Mexico City, Mexico — Some 2.2 percent of all U.S. gun sales are made to smuggling rings that take firearms to Mexico, a scale of illegal trafficking that’s “much higher than widely assumed,” an academic study released Monday found.

An average of 253,000 weapons purchased in the United States head south of the border each year, according to the study by four scholars at the University of San Diego’s Trans-Border Institute and the Igarape Institute, a research center in Rio de Janeiro, Brazil.

Profit margins at many gun stores are razor thin, and thousands of U.S. gun vendors would go out of business without the illicit traffic to Mexico, said Topher McDougal, an economist educated at the Massachusetts Institute of Technology who’s one of the study’s authors.

The study’s conclusions are likely to add to controversy over what role U.S. weapons smugglers play in Mexico’s drug violence. Mexican officials have long blamed lax gun laws in the United States for the availability of weapons in Mexico, which has only one gun store and considers gun ownership a privilege, not a right.

The value of the annual smuggling trade is $127.2 million, says the study, “The Way of the Gun: Estimating Firearms Traffic Across the U.S.-Mexico Border.”

The traffic is reflected in the disproportionately high number of federally licensed firearms dealers along the U.S. side of the border, said Robert Muggah, another of the four scholars. Of the 51,300 retail gun shops in the United States that hold federal licenses, some 6,700 of them are concentrated in the four U.S. states that border Mexico, Muggah said. On average, there are more than three gun dealers for every mile of the 1,970-mile border between the countries.

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