This is a testament to federal gun laws here in the United States. According to a series by the Christian Science Monitor, drug gangs in Mexico get most of their guns from the U.S. Why? Because it's easier for them to get weapons in the U.S. than it is in Mexico. From the article:
Lax gun laws and lax enforcement in the United States have made it easy for Mexican gunrunners to buy and transport everything from AK-47s to Stinger antiaircraft missiles, which then allows the cartels to use these high-powered weapons against rival gangs or against a military attack. More than 90 percent of the thousands of guns confiscated yearly in Mexico have been traced to US origin.
It gets better:
Most alarming is the increasing flow of combat-style rifles into Mexico, often just a few at a time hidden in the trunk of a car. That trend is partly a result of Congress allowing the US ban on assault weapons to lapse in 2004.
But the Bush administration is already on top of it:
The Bush administration has waked up late to Mexico's gun problem. Last year, Homeland Security Secretary Michael Chertoff told Congress he didn't know where most of the confiscated weapons in Mexico come from.