Mexican drug barons are outgunning government forces on both sides of the border, thanks to a torrent of smuggled American weapons and ammunition. Middlemen acquire weapons at gun shows and stores in the United States and then ship them south.

Thomas Mangan, a special agent in the Phoenix office of the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), describes the fast-growing trade as an “iron river of guns.”

The drug cartels’ weapons of choice include the Colt AR-15 .223 caliber assault rifle, the AK-47 7.62 caliber assault rifle, and FN 5.57 caliber pistols, according to William Hoover, assistant director for field operations at the ATF, who discussed the issue before a House foreign affairs subcommittee. Gangs sometimes use rocket-propelled grenade launchers against government forces, he said.

“You’re looking at the same firepower here on the border that our soldiers are facing in Iraq and Afghanistan,” Mangan told Security Management. “A 50-caliber weapon would tear apart a low-flying helicopter or a Border Patrol vehicle. These guns are being used by people who know guns, who have military training. They are fighting their country’s army.”

More than 5,000 people have been killed in Mexican drug violence over the last two years, including soldiers, police officers, journalists, bystanders, and gunmen working for rival gangs. The drug barons have hired small armies of gunmen as enforcers. They battle government forces and rival gangs in vicious turf wars. The most feared gunmen are the Zetas, all former members of the Mexican military’s special forces.

The 2007 antidrug program known as the Mérida Initiative provides $306 million in U.S. funds for counternarcotics, counterterrorism, and border security. But efforts to crack down on the weapons trade are in trouble.

The intelligence and evidence-sharing network with Mexico has been “overwhelmed” by the flood of weaponry, Hoover said. The United States is responding by hiring more agents and equipping law enforcement on both sides of the border with advanced detection and tracing technologies.

In January, the ATF announced it would add 35 special agents and 15 investigators to the southwest border under its Project Gunrunner, which is intended to halt the flow of black market guns. It is deploying retrace technology to nine U.S. consulates in Mexico to help investigators trace the origin of seized weapons.

The United States also deploys gamma-ray scanners, density measurement devices, and commodity testing kits to help prevent the cross-border movement of illicit drugs and firearms, David T. Johnson, assistant secretary for international narcotics and law enforcement affairs, told a Senate committee in an earlier hearing on narcotics trafficking and organized crime in Mexico and Central America.

Senator Jeff Bingaman (D-NM) has introduced a bill to increase funding of Project Gunrunner. But some observers say that U.S. gun laws prevent effective interdiction of weapons. “It’s difficult to detain [suspects] and difficult to look into a gun store. It’s difficult to prosecute,” says Maureen Meyer, associate for Mexico and Central America at the Washington Office on Latin America, a think tank.

Still, the ATF indicates that it is making headway. In 2007, it seized 1,297 firearms and recommended 465 defendants for prosecution as a result of Project Gunrunner, 52 percent more than in 2006