Source of Illegal Mexican Firearms
"Military-style weapons, such as high-caliber and high-powered semi-automatic assault rifles, are readily available for civilian purchase in the United States. Many of these are imported from former Eastern bloc countries and then can be bought by straw purchasers and transported to Mexico.
"Of the 87 percent of firearms traced from Mexico to the United States from 2004 to 2008, about 68 percent of those firearms were manufactured in the United States, and about 19 percent were manufactured in third countries and imported into the United States before being trafficked into Mexico. Many of these firearms are high-caliber and high-powered, such as AK-type semi-automatic rifles and AK-type pistols that are becoming increasingly popular with traffickers."
Feinstein, Senator Diane; Schumer, Senator Charles; and Whitehouse, Senator Sheldon, "Halting U.S. Firearms Trafficking to Mexico," United States Senate Caucus on International Narcotics Control (Washington, DC: June 2011), pp. 12-13.